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.
Sanguimancy can be a very potent magical style to be sure but like Necromancy (which is what I do) it is taboo which means socially and culturally you may receive negative attention for both by normal people and law enforcement.
That also means that if you aren't careful with this left hand stuff it can teach you a lesson of tough love.
Am I saying don't do it? No. If you're really into blood that much then why not? After all blood magic is pretty straight forward and easy to do. And it gets results.
Blood magic isn't bad, it's just more unorthodox.
Just make sure you're careful and you use common sense just like with any other style.