Are you absolutely sure he is true black? Perhaps now that it's summer he is showing his real colour - could he be brown?
The pony I bought late last year was pedigree registered as "black". However, now his new coat is in he is most definitely seal brown. No amount of vinegar or any other substance is going to change it.
I think Forum members have recommended adding paprika to the ration to prevent black from fading. No idea how much, or how early in the season you would need to start - it's on here somewhere!
Pictures would help