Originally Posted by redshift1

Typically the black greases I have bought over the years have been that colour due to the presence of the chemical molybdenum disulphide ( "moly" ). This is a particularly low friction substance so these greases are more "slippery". A very common application is in constant velocity joints in autos. ( Though I would imagine graphite containing greases are also black / grey. )

I would suggest it's not going to make a lot of difference to the average mechanic whether you use this or not.