Nothing. Black Lion chests aren't worth it. You get an illusion potion that makes you look different but can't actually do anything in combat. You get a consumeable, that 9 times out of 10 is useless (+10% exp per kill, yeah, kills give next to no exp to begin with). The only time the third item is of note is when it's another key.
20 bucks at most. Charity or not. Ive gotten a key from nearly every chest Ive opened and Ive been around since head start. I have had to buy only 2 keys so far from BL. I dont usually have good luck on games in regards to finding epic "loots", but I always get a key :P
He who needs a pat on the back makes every man his judge