Since the introduction of the Slayer Helmet, the black mask has fallen into obscurity. There is no reason currently to use a black mask over a slayer helmet outside of purely aesthetics.
My suggestion to fix this issue, is to remove the 10 defence requirement from the black mask, but keep this requirment for the slayer helm. This would give the black mask a niche as a slayer item used by low defence accounts. It would not disrupt game balance in anyway but would bring the iconic black mask back to its glory.