There is a bug involved here, but it isn't actually with Black Pearls. High Quality results for gemstones don't double but normal quality ones do. I haven't tested Blood Rubies so it's possible that they fixed that when they fixed the ability to donate HQ stones to the stronghold coffer. BR +1 can now be donated while Black Pearl +1 can not.
As far as the crafting, if you get a HQ result on Black Pearl it doesn't double but the normal one does.
"We have always been at war with Dread Vault" ~ Little Brother
The high-quality aspect hadn't occurred to me. Seems correct. Yet another problem with that poorly designed "+1" professions change from years ago. Also dab hand seems to override the 2x event logic - when it occurs you only get 2 gems not 4.
Additional bugs not directly related to black opals or 2x event. The RNG for professions is just completely messed up. With 0% (ZERO) focus, there is still a chance to make a high quality (+1) item. And with 93% proficiency, 3 failures out of 4 attempts.
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As far as the crafting, if you get a HQ result on Black Pearl it doesn't double but the normal one does.