Maker - Alchemy Skill FAQ and Walkthrough

According to JMS, no. Both black and dark crystals effect STR, INT, DEX and LUK according to the following chart:

Crystal	Low Range	High Range
Low Black	-1	1
Mid Black	-2	2
High Black	-3	3
Low Dark	-2	2
Mid Dark	-3	3
High Dark	-5	5

I'm aware this is slightly different from Isaac's explanation of the dark and black, so either one of is wrong, or it's implemented differently between versions.

To answer the root of your question - Only sapphires and diamonds are restricted to weapons.

I'll go with your description because mine has a much more tenuous source. Does seem to make Black Crystals rather worthless though when you can get a low dark much easier than a mid Black.