T-Moon sand is perfectly safe for use with Geos. Actually, the grain size is small enough for the to sift it out of their gills as is their natural technique (rather than s[otting back out as they have to do with gravel).
It all comes down to preference. Since Geos aren't generally found on black sand, I'd choose to use a more natural looking substrate like PFS. Same performance and CHEAP!
Go to cichlidforum.com for lots of info about ColorQuartz.