Do you mean spells that USE blood, or "blood magic?"
"Blood magic" as a discipline is a Hollywood trope, and it's very dark and sinister and dramatic, just the way us left-hand pathists like it. Unfortunately, it's not as flamboyant in real life.
In practice, the use of blood in spells is a divisive subject. Many Wiccans refuse to use it because they believe it disrespects their bodies, their sacred vessels. Others permit its use, but NEVER in dark magic workings.
For the rest of us, a drop of blood here or there is an excellent way to accelerate your spell potency - but only if done safely.
For example, you should NEVER draw more than a few drops at a time. A small papercut-like incision on the fingertip, locally anesthetized with an ice cube, and cut with an implement sterilized in alcohol is all you'll ever need.
You should also be VERY careful when handling blood - yourself or anyone else's. Dispose of bloody ritual remains by burying them, burning them, or throwing them into a waste receptacle that's been properly marked as containing biohazardous material. NEVER handle anyone else's blood -- but if you must, wear gloves.
Be safe, be sane, and be sensible.