I'm looking at a medieval trade manifesto and it says that they are carrying "black oblillnum". Does anyone know what oblillnum is? I'm guessing from context that it is a plant-derived thing, like a flavouring or medicine.
A genuine mystery! 
Got my curiosity seriously pegged, but not actually knowing what it is I'm not of much help... 
I did do a bit of rumaging about, but I suppose you've already followed the trail to Socotra and/or dragon blood trees - especially since I seem to recall that you have a passion for those trees - or maybe this is in fact whre your trail started... In any case I havent dug anything worthwhile up and nothing conclusively seems online on the subject, but I would consider ordering home a couple of books on Socotrian history and fauna or get in contact with someone knowledgeable in either Arabic history and/or Semetic languages might be the way forward if nothing turns up here. Personally, if I were using it for inspiration for a Ars story (and not for genuine research), I'd go with linking it to oils from either an aloe or cassia if not the dragon blood tree itself.
BTW, I do seem to recall something with oblil-something meaning forgetfulness in some language, but can't remember any specifics I'm afraid.
Good hunting - and hope to hear if you find more on the issue.
Oblivion?
Anyway, ran it through my whole batch of search engines and no go. (pity C4 and GoTo engines are both goners, good for odd searches sometimes)
Ok, no luck from any of the encyclopedias here... English, German and Latin dicitionaries also gave zero.
And none of the online dictionaries i have bookmarked gave anything at all either(tried spanish, portugeese, arabic, turkish, greek etc etc, just about any i could)...
Is it a creative spelling of olibanum?
Which is a type of frankincense (or maybe just an alternative word for frankincense) used as medicine and incense?
Ah...that could well be it. It's mentioned next to myrrh in a manifest, and so it would fit. The manifest also mentions frankincense, though.
Ah, I have it now: black olibanum is incense (which is called lubban in Arabic, which means "frankincense" literally) made from a related but different tree so that the resin is black rather than the pale orange colour of high quality frankincense. So, you are right, its a close relative of frankincense. It fits.
Cool.
Thanks all. I will now vanish back into my PbP, to plot and scheme.