Q:
free level from a chest
A:
How rare are the free level scrolls from the black lion chests?
Low enough that it does not register on the drop rate research. So I would say it would be less then 5%(vary rare chance so rare it can be said it almost never happens).
In Game: Storm Bluff Isle — Anzz, Anzenketh Kyoto
The answer to that is, don’t spend money opening BL chests, it is a waste. :P