As soon as you see any blood stop pulling and you wont get much blood if you are in a vein. Then you have 2 good choices 1. Pull the needle out and try another spot or 2. Pull the needle out just a little bit and pull back on the plunger again to see if you are out of the vein.
The blood in the syringe is fine. Its sterile unless it gets into the air which it wont if it stays in the syringe.