A lot of people blame this issue on static, this isn't what's causing the problem. If your printer printed fine before this issue, then static isn't the problem. Overspray is rarely a static issue when your printer has been established, and printing fine in the same area for a long time.
Check and be sure your printer is clean, swab the head, etc. (it won't help, but it helps you to feel better
). If this doesn't help, then more than likely one of your heads is going out. Black uses a lot of different colors to create the look, so it will start to shadow/overspray in areas that it shouldn't be. Finding the head that's going out can be tricky. I have replaced numerous heads because of this shadowing issue.
Slowing your printer down won't help the issue. The printer is very capable of printing small text at high speeds.
Based on your image, I would say that your black head is out. It will creep up on you and it won't go away, no matter what you try. Do not try to overclean it, it won't help. Based on your image, this is a tell-tale sign that the head has had enough production and it's time for replacement.
Your image shows the exact problems I had. I tried everything in the book. Switching cables, cleaning cable ends, swapping cap tops, dampers, etc. etc - nothing fixed it, but a new head.
The head starts to lose it's accuracy as it's life is close to over. The first signs are when it's starts this shadowing effect.
Test printing in service mode will help you figure out which head it is, but based on your image, the black head is going to be my best guess.