beside Protection etc. probably only useful for PvP .. where you will get hit. Otherwise you should be more focused on not getting hit.

So similar direction as Gert suggested just a question on how well you manage mana without it ... Int. gems influneces a bit mana, mana regen as well as damage ... but not to the same degree as pure damage gems for example.
And also depends on how many black gems you got ... is it 3 or so ? I probably would balanced number of magic damage and Int gems for the blue and then get magic damage for the black ones.