I have gotten a couple blakcs in a single run before. I was soloing an Ascalon mission with my lvl 20 ranger and a charr dropped one and a charr boss dropped another. Most of my black dyes have been from Charr(as their blood is supposed to be black, this makes a lil sense.)
One thing that I have been fairly lucky with is black dyes. Been playing since a week after release and have had about 2 dozen black dyes drop, from assorted mobs.
Lets see... the last 4 I remember were: A Charr in Pre-Searing, A Madrangor Sliter in Jahai Bluffs, a Crypt Wraith in the Cathedral of Flames, and a Snow Wurm just north of Boreal Station. That's right, a Snow Wurm. The guys who don't even attack you unless you're about 2cm away from them. :-)
I just got one drop a mergoyal a few days ago. Earth ele another time, and i forget all other times they've dropped for me. I've obtained 4 or 5 total from drops. ;-)
I really don't think it is luck as much as it is the number of kills. Everything can drop a black dye so the more you kill the more likely you are to get a black dye.