Witchipedia

Blood magick is just magick using blood as part of the spell or ritual. It can be used in a number of ways. As a taglock of course, and some believe it to be an extremely powerful taglock, perhaps even resulting in an unbreakable spell. It is particularly useful in binding spells, especially to bind objects to you or to bind one's word to an oath (sometimes used in handfastings and initiations). In cases of spells involving invocation or evocation, it may be used as an offering. Apparently, some beings consider it a very high-value offering, but others might find it offensive and there's always the risk that someone might get the wrong idea. (Blood sacrifice, using animal blood as an offering to the dead or to Gods, I think is religion rather than magick, so I'm not going to get into that.) Many witches who grow their own herbs like to use blood as an offering to the plant ally spirits of their gardens to bind the plants to their own energies to make their magick more effective. And, of course, the plants also like nitrogen.- Though I'm not sure that counts as blood magick perse. Many witches also use their blood to consecrate their tools and bind them to themselves. Blood is also useful for manifesting servitors. Blood, as a symbol of your very essence, can be used as a carrier of your Will during spells of manifestation of all sorts. It comes, pre-charged, if you like. Another possibility is the use of bloodletting as a means to raise energy- in fact, if you're using your blood as a tool of manifestation it will work best if you acquire the blood this way rather than just scraping up an incidental drop and finding a use for it. And, of course, you could do it with someone else, all consenting adults taking all advisable precautions against blood born pathogens.

Black magick is generally defined as magick that causes harm, either by its effects or via the methods and materials used. Blood magick would only meet this definition if it was used to harm someone or if the blood was taken from an unwilling donor. Most blood magick applications require your own blood, but not all. Anyone else's you use should know what you're up to and be fully on board.

With regard to dangers; You'll want to take the usual precautions against infection, of course. Bloodletting itself can be addictive and that can cause problems as well. Also, when using blood to feed a spirit or spell object, well, there's the theoretical risk that you'll end up needing to feed more than is good for you or that the spirit can use the blood to gain power over you. Likewise, if you are doing blood magick with another person you could be allowing them control over you by giving them access to your most powerful taglock.