The skin on some people's fingers is just too thick, I had the same problem. You can take the blood from somewhere else on your body and it is just as valid. Wherever is easiest to get it from. The blood is the slower vibrational equivalent of the prana lifeforce energy, so we put the drop of our blood to physically connect our soul into the dedication prayer. This is the purpose, so it really isn't specific where you should take it from if that location is too difficult.