Left hand black key ascending glissandi

[pies]

How?  I can't do these without making myself bleed..

[pianistimo]

is this a halloween topic or something?  use your rh palm down instead of fingernails to the keys.  it's a matter of pride. 

[qoppa]

I use my thumbnail.  It hurts, but I usually don't bleed...

EDIT: I completely missed the part about it being a black key glissando.  A thumb probably won't work too well in that case...)

[guendola]

Don't try to push the keys down. "Glissando" means "gliding" and that is what you are supposed to do. People also tend to glide way too fast. Just like any other normal note, a glissando has a duration.

So apply as little pressure as necessary and glide on two or three fingernails down. This needs some experimenting, start with almost no pressure at all. For the other direction, use the fingernail of the thumb.

[mad_max2024]

I usually prefer to use the fingertips palm down like pianistimo said. I find it gives me a bit more control over what I play. Never bled during one.
I sometimes also use the side of my index finger if the piano/keyboard action is light enough.

But that depends a lot on the piano, it is easier to make glissandos on some pianos rather than others and if the piano has sharp key edges I would not find it unlikely you would bleed. If I found one of those I would either use the fingernails or simply skip the glissando, I usually do them on pop/rock music so it's easy to go around them. Never played a classical piece with glissandos.

Since pies mentions a black key glissando I don't think using the fingernails would be very doable, I would use the fingertips. Just make sure to keep the hand at a low angle so it glides through the keys like guendola said, if you raise it too high you will bump into the keys instead of gliding.