User blog comment:SkyEyeKiller23/Black Blood Arts?/@comment-27188908-20131208182217/@comment-30011748-20131209021555

Yes, some of them have conditions to be used (Example already given:Buster Blade blood art,  CC-The End, activates only in level 3 Burst Mode and takes up 75 stamina, almost a whole regular Stamina bar. These black-icon Blood Arts usually will have some sort of drawback (like slower skill action) in comparison to their gold-icon Blood Art alternatives at the benefit of more power.

Some of these black-icon Blood Arts are hard to obtain too, like the Buster Blade's second black-icon Blood Art, Last Revenger, which does not have a Level three Boost condition (only boost levels one and two).

An efficient way to level up Blood Arts is to equip the Blood Arts Growth Up (Sword)  Part and fight against Ouroboros which have weak tentacles to Sunder and Pierce attributes. Its legs, when unbound or in enraged state will be weak to Crush attacks though there are other Aragami that are weak to purely Crush attribute available for battle (if using certian Blades or Hammer).