I am looking for suggestions for a portrait lens for a 70D. I currently have the 50 and zoom lenses.
Which 50mm lens? What zoom lenses? Use the zoom to decide what MIGHT be a good prime to purchase to use for portraits. I emphasize MIGHT because you can get pretty good results from a quality zoom. I'd only invest in a top shelf prime lens for shooting stills. Quality zooms can do nearly as good a job, especially with a 70D.
On a full frame, or 35mm, camera, the consensus, "ideal" focal length for portraits around 85mm. This focal length is good for the depth of field that it can create. A longish lens, plus a relatively short distance to the subject can add up to great OOF, out of focus, backgrounds. See this chart.
Use the 7D setting to approximate your 70D. Use the 5D setting to approximate a full frame sensor camera.
Focal lengths to use for portraits can vary widely, depending upon your distance to the subject, distance to the background, and how much background blur you're looking to have. Most people seem to like focal lengths around 50mm for an APS-C sensor body like your 70D, which would give a field-of-view equivalent to 80mm on a full frame sensor body.
But, you already have a 50mm lens, though. I would think that focal lengths ranging from 50mm to 135 mm could work well as a portrait lens on a full frame camera. That works out 31.25mm to 84.375mm on an APS-C camera body. An EF 24-70mm zoom lens is fairly close to that range. An 85mm prime would be at the top of that range, and could be of limited use as a portrait lens.
Without knowing what lenses you have, how you wish to frame the shots, and what type of distances your shots would involve it is hard to give good advice. Too many unknowns. I can only present food for thought.
However, there is one piece of advice that I can give now. Use a very robust tripod and head, as well as a remote switch, or your 2/10 second shutter delay in your camera. A good, strong tripod head will help reduce any minute vibrations from the shutter activating. Believe it or not, it does make a difference, most especially with longer and longer focal lengths.
The classic 85-135 portrait length for FOV is like 50mm to 85mm on a crop sensor. If you have the room to back up then going longer is not a bad thing because it can blur your background even more.
If if you like your 50 then consider maybe getting a 70-200L in one or another version. They do double duty as portrait and sports and general use. Well that is triple duty. The mighty 70-200 f/2.8 IS L mk. 2 is the top of the heap and is a lot less expensive now at $1700 than when I got mine for $2500 but it was a great buy back then too. If that exceeds your budget the old version 1 is still very good.
If you don't like your 50mm then either an upgraded 50 or an 85 is great.
Another idea would be a 100mm f/2.8 macro. Either the L version with IS or the non L. These are very sharp lenses with a bright big aperture and of course you get double use with the macro capability.
Both of these statements are true.
"I'd recommend the Canon EF 85mm f/1.8. It's a really good portrait lens at a very reasonable price."
"If you have the room to back up then going longer is not a bad thing because it can blur your background even more."
I think if I were to buy a purpose built portrait lens for a crop sensor 85mm would be tops in focal length. Your 50mm may be ideal. Personally I use a full frame camera with the 'migthy' ef 70-200mm f2.8L II as my go to portrait lens. And this is from a guy that owns the ef 50mm f1.2L and the ef 85mm f1.2L. The two lenses that are considered the best there is for portraits.
Don't get me wrong they are truly fantastic lenses but as I age I have gotten lazy. The zoom is just too easy and does a wonderful job.
Forget the tripod idea. Bad suggestion. If you use it you will never get that great portrait shot. You have to be mobile and flow with the subject, especially with females.
Check out that 85, you will love it.
You're getting lots of good advice, but I'll 2nd what Ernie says...
My favorite lens for portraits... tends to be my EF 70-200mm f/2.8L IS USM (even though I own the EF 135mm f2L USM on a full-frame body).
I have, just occasionally, even used my EF 300mm f/2.8L IS USM (that's a prime lens, not a zoom) for portraits, and they're fantastic.
You typically want to avoid short focal lengths for two reasons (more on that in a moment) and also typically want a lens with a very low focal ratio AND well-rounded aperture blades (more on that in a moment).
For a 70D, a 50mm focal length would be a minimum ... I wouldn't go shorter than that. And some desirable things start to happen as you increase the focal length. Also, not just "any" 50mm lens will do... the size of the aperture and the shape of the aperture blades will effect the quality of the blur in out-of-focus areas. This can create a very beautiful background which provides a suggestion of surroundings, but the deliberate blur means your in-focus subject will pop and you'll get better separation of your subject from the background.
Canon has the older EF 50mm f/1.8 (non-STM) lens, which had a 5-blade aperture and the aperture blades were not well rounded. This created a background blur that wasn't very smooth. It had a jittery look to it.
Canon has since replaced that with the new EF 50mm f/1.8 STM lens which now has a 7-blade aperture and the quality of the blur is much better (not as good as the EF 50mm f/1.2L, but pretty good.)
The "L" series lenses are the best lens options you can get from Canon, but some non-L lenses are pretty good too. For example, the EF 85mm f/1.8 USM is a very good performer. The EF 85mm f/1.2L USM is a better lens for portraits (and ONLY for non-moving subjects... while the optical quality of the portraits from this lens are gorgeous... the focus motor speed is slow. So if a subject is moving you'll have more missed-focus shots.)
A short focal length lens (wide-angle) will distort the image and tends to end up in a very non-flattering look. I have seen these lenses used for portraits, but a photographer really has to know what they're doing and even then the wide-angle effect is used sparingly. Also short focal length (wide-angle) means you get a naturally shallower depth-of-field and the degree of background blur is reduced (depending on how short the focal length is, it can get reduced to almost nothing.)
So between the broad depth-of-field and the wide-angle distortions, short-focal length lenses are usually not used for portraits.
Longer focal lengths lenses that can ALSO provide a low focal ratio are highly desirable. Lots of good things happen... the depth of field becomes shallower, the blur intensifies, if the lens has well-rounded aperture blades then the quality of the blur is smooth, and since the angle-of-view is narrow it helps reduce the area visible in the background (and this is why some photographers love shooting with a 70-200mm f/2.8 zoom.) If the zoom doesn't offer a low focal-ratio throughout the zoom range then it's usually not good for portaits.
The one bad thing that happens when you use a long focal length is that YOU have to step back much farther to get your subject in the frame (but when you do get them in the frame they usually look better.)
Just be warned that while it's often possible to step back far enough when you are OUTDOORS it may not be possible to do this if you are shooting INDOORS. That alone can be a reason to go for a 50mm focal length lens.