Blood spells should be taken seriously and not just because they sound cool. You have to understand that when using blood magic, it's not a kiddy joke. Your blood is your life force, it is you. If you're using blood as an offering, there is nothing more valuable you can offer that your life essence.
First of all, you want to be extremely careful, and use sterile items. Don't just use an object that you found around the house, you don't know how the item was previously used. Just be safe and buy items from your local store or whatever. Be careful when choosing where to draw the blood, even a little can be potent.
Just be super careful if you plan on preforming a blood magic spell.