ANSWER - It depends on how it is used. I think it is important to be absolutely clear that the use of blood is not a necessary part of what it is to be a witch. Many witches (especially Wiccan witches) do not work with blood at all. This is a personal choice and certainly does not make an individual less of a witch.
Blood can be used for many purposes in spell work. It can be used to bring individual energy to a working, it can be used to invoke protection (I know more than one witch who has a drop of her own blood in a locket worn by her child) and it can be used as a sacrifice or offering when calling upon a higher power. Blood can also be used to forge and bind a connection - I, for example, have bonded my ritual knife to myself with the linking of my own blood (this is a practise no Wiccan witch would ever get involved in). Blood has a wide range of uses and individual purposes and - distasteful as the use may appear to a non witch - the benefits can most definitely outweigh any initial squickiness.
In terms of whether the use of blood can be dangerous. Yes it can be. There are three primary ways that I would caution against using your own blood. The first is using your own blood to bring life to a poppet or doll that may fall into the wrong hands. If you share your lifeblood with an inanimate object with the intention of giving it "life" be aware that somebody who knows what they are doing could use that object to cause you direct harm. If you invest your blood into anything, ensure you are in a position to keep the object secure. If in doubt - don't.
The same warning goes for using your own blood as a taglock in spells. I've heard of a few cases where a witch has used her own blood as a taglock for hexing in her own bloodline and let me tell you, that isn't a great idea at all. Messy... For something like that you'd be better off using a photo or even a name written on a piece of paper as a taglock. Keep your own blood out of it.
I'd also suggest that anybody who doesn't comfortably know what they are doing avoids the use of blood in invocation. Don't offer your blood to any spirit/demon/entity who asks for it in exchange for their assistance. There is a school of thought that using blood can tether a spirit to you and while I find this to be a bit fantasy novel thinking for my tastes it isn't worth the risk if you do happen to encounter anything wanting a blood sacrifice. (Summoning in general, blood sacrifice or otherwise I'd avoid to be honest.)
In summary - blood workings may often be stigmatised by the general public as "dark magic" but in truth blood is just another magical ingredient readily accessible to the witch. The dangers of using blood come from creating a link to yourself and not from any danger intrinsic to the blood itself. Whether you choose to use blood or not is the choice of the individual and each witch will make this decision for herself.