A blood bond is a very deep and hard to break bond that is done obviously with blood. There is one on one blood bonds or group, house and pack type blood bonds as well.
There are pros and cons to one.A lot of it is dependent on how it is setup. It can be setup so one is the master of it. It can also be setup so its shared in the group equally. Outside of the obvious blood sharing dangers of the ritual a couple other dangers exist. One being if they are doing the ritual and in charge of it all they can influence and control you somewhat.
Somethings one can expect from a blood bond: Sharing of thoughts, feelings, urges, traits, quirks, feeling others tied to it, increases in ability, possibly strength and speed as well. Mental aspects can increase as well.
A lot of the pros come more from what is gained/shared and how it makes one feel more like family, closer tied and whatnot.
So outside of the normal dangers, and possibility of how it is setup how does one do a blood bond? Well there are many ways actually. The ones that work best have a bit of a ritual involved to help raise the energy and give focus. An example of a blood bond would be this:
A small group has written and agreed upon the words for a ritual and set aside the space, time and all to do so. They all enter, the person(s) who is the head take the lead, starting the ritual then saying the words. The one taking the bond steps up, either repeating what was said, or saying what else has been decided upon. After this they share blood. Thus completing the blood bond and adding a new person into the family.
That is the most basic way to do a blood bond. Indeed some folks do think it is gibberish or pointless. Though I have found it does have a lot of affect on a group and how well they share and work together as one.