Hi! I'm not a medical professional, just the parent of a kid with diabetes. A drop of blood just means you came close to a larger blood vessel with the needle. It happens, but you probably need to try to avoid that area next time.
There are spots on my daughter's legs where we know we can't give a shot because it will bleed. The shot is intended to hit the blood stream as fast as possible, so it's not necessarily a bad thing, it just can cause discomfort and bruising. You also need to watch that no insulin came back out with the blood, because it could mean that the entire dose didn't make it into the system.
In short - nothing.
Basically what happened is that the syringe hit a blood vessel on the way in or out and that burst. No big deal. It will happen occassionally with any shot not just insulin. No harm will be done. This is fairly normal.