It would be no more or less risky than drawing blood for any other reason. IE; Infections from misused or inappropriate implements or through lack of proper care to the wound afterwords.
Though you should also consider ootential harm to the crystal as well. Some, like quartz, are very durable, non-porous, and non-reactive. However other crystals might be very porous (like selenite or pummice) and so the blood would stain in and might leave a permanent mark. And some crystals are weakened by moisture, and might get surface pitting or other wear. ...If you wouldn't use water on a crystal then you shouldn't use other liquids either.
Would anything bad happen if you put your own blood on a stone or crystal, is it safe to do so? Or would something bad happen, I'm not going to mention the mundane stuff that could happen because I think your question is more magic based then mundane based but correct me if I'm wrong. I believe one way to explain this is to first explain what blood magic actually is, since this sounds like your asking about blood magic. First, blood may be used in spells however it is not a style of magic. An example of blood being used in spells can be in the form of a taglock, it can also be used as a power source such as life force, but you still need a working to add to that power source, it does not necessarily make a spell “more powerful” as it is a component. And just for the notice here, You need to be incredibly careful when using blood magic for the physical safety related aspect of blood magic. Now it sounds like your looking for a simple spell that uses blood, maybe you think it's "cool" to use blood in spells cause movies and tv shows always have witches use blood, because who knows. But Simple in spells don’t really go together. As all spells require the mastery of the basics of witchcraft to be used, otherwise your just , as you have said, putting blood on a stone. It sounds as though you need to look into the beginner level stuff first, ie:
"Before You Cast a Spell" by Carl McColman
"Spells and How They Work" by Janet and Stewart Farrar
"The Veil's Edge" by Willow Polson
"Modern Magick" by Donald Michael Kraig
On Spells of magic .com you can find these that can help explain things:
