Color.
Sorry, I'm trying hard to to be a smart ass, but other than the color (clear, transparent blue, or opaque black) they're all the same. Two 7-mil plies with oil in-between.
Black heads are more expensive in general because (I'm guessing) it actually takes quite a lot of black dye to make plastic totally opaque black.
The Hydraulic Black is actually 7.5mil black and 6.5mil clear. This black film is pigmented, not dyed. The Hydraulic Blue is two plies of identical 7mil blue. The blue is a bit more open/resonant, whereas the black is the fattest-sounding, with the shortest sustain.
Cheers!
Hmm, so the Black Chromes are basically Hydraulic Blacks with a .5mil thicker clear ply (according to this) and no oil? Interesting.
It's not quite that simple. The 7mil clear film is very different from the 6.5mil. In addition, the 7mil clear ply of the Black Chrome is on the top, as opposed to the Hydraulic that includes it on the bottom. There is much for to films than just the thickness