Black is almost always pigmented with carbon black. Carbon black is extremely hygroscopic. If the material is way overloaded or poorly compounded it could have a bigger impact because of its relative concentration or poor dispersion (possible increased surface availability). Sometimes it will be overloaded to hide other ingredients (like other colors of regrind) as others have mentioned, sometimes it’s through ignorance, or it could be loaded heavier to compensate for insufficient mixing in a large extruder.
If the increased hygroscopy in your black is unique to that color, consistent across production batches, and is made from the same base resin, I suspect this is your issue.
Alternatively, there could be other differences that have symptoms similar to moisture absorption, but which could be caused by too much regrind mixed in, which in many plants will only be put in the black since carbon can hide it. Take a look and see what claims the filament manufacturer is making about what is or isn’t in the plastic in the way of resins, and if there’s any question try something from a different manufacturer so you’ve got a good basis of comparison.
I hope that helps somewhat. Best of luck. If you learn anything, please share it!