if it wwas Carbon Black, I have had the same problems with that with the regular Golden fluid acrylics in marbling, so i was going to switch to Mars Black. Someone told me to try adding a little bit of thalo blue to it to see if that would solve it.
Yep, Carbon Black. I found that even adding a bit of gall or water, then reducing the amount of black used fixed the problem while maintaining the quality of the colour. You really don't need to slather on this black. Interesting about the thalo blue - I haven't tried that but I would. Thanks for the comment!