That's +1/+1 counters, which are positive. +1/+1 counters can be negated with -1/-1 counters. The card could have said "whenever a +1/+1 counter is put on a creature an opponent controls, put -1/-1 counter on that creature." and it would undoubtedly be black. Preventing counters isn't black, but preventing +1/+1 counters is.
Why would ramp be white just because it used the sideboard?Ouch. Dunno. It seems that they are moving towards a point where some colors will get some kind of other answers. Maybe white will get a ramp that works with sideboard.
White already has catch-up ramp.
I can see ransom being a thing in some form.
Xyru Specter I could kinda see happening.
Lightning Colt obviously functions, so I can see it being printed without the pun name.
Generated Horizons I don't think, but I do think land tokens would be cool.
Growth Charm that wrote out the effects.
Zyym I could see getting printed.
Unicycle's mechanics I can see.
Enchanted Prairie could maybe happen.
How would that even work? I die when I have 5 life and 5 corruption? How is that different from having 0 life and corruption not doing anything?I was with you until this part. I think I like corruption counters that raise your life floor. I.e. work as a more permanent form of damage while giving white and black options to tweak it or remove it.
Your mechanic shouldn't have very much specific hate. You won't be able to remove your corruption, there would be maybe a white rare that prevents counters on you and maybe your creatures, and maybe one that prevents counters this turn. There is no need for more hate.