Patience is definitely a virtue when assisting. One thing I've learned is to only take small pieces of shell off, without breaking/tearing the membrane. Once I have an opening in the shell, swabbing a damp Q-tip on the membrane makes it turn transparent... and blood veins easy to see.
Hopefully the little guy didn't lose too much blood. Chicks are tough little critters. Just start it on electrolytes to give it a boost as soon as it goes into the brooder. Best of luck with him!!!
He's still alive, though I'm letting him finish absorbing the rest of his blood. His brother just finished hatching and is healthy and lively.