So I had this same issue on my MG7720. I did the clean a couple times and got the color inks to all print fine but the standard black was barely printing anything. Replaced cartridge, ran the clean process, and tried to standard black text printing. I was able to change the paper type to high resolution paper and yes that does work and uses the small black ink cartridge.
But I was able to get the large black cartridge to print again by taking out and cleaning the print head. It snaps right out and back in again. Take out the cartidges and pull the front latch back about 1". You can look up in youtube. I ran it under sink with very warm/hot water and ink that is in there comes out. Turn water down low so it doesn't splatter all over. Then soak the print head in mixture of warm/hot water and some windex or similar for about 10 min. And then rinse it off again and dry it slightly with a paper towel. I snapped it back in, put the ink cartridges back and the black prints great again set on regular plain paper. I'm going to be sure to print something at least once a week so these don't dry out and clog again.
