Blood can be used for several reasons, but it would depend upon the working.
For example: It could be used to charge a sigil, in a love working, as an offering to an entity, etc.
It is a connection to you, since your blood is your vital force and is believed to give more power or strength to a working.
Blood is a way of forming an intrinsic connection between yourself and the magic you are wanting to work. On a mundane level, Blood contains your DNA (and with it your ancestry), carries oxygen and CO2 and hormones and fuel for your body to use to run. Philosophically, blood is the representation of the self, and connection to family and/or nation. So it is literally and figuratively an act of putting yourself into your working. Blood is your signature, and the essence of who you are as a person and as a sampling of the energy you carry as a part of your physical being.
As such, anything involving use of your blood is placing a very intimate connection between yourself and what you are trying to do. Most often this translates into connecting yourself and your energy to the fueling of the spell. But if it is being used as a form of sacrifice it is also a symbolic act of giving of yourself and your energy to a particular being or Deity as a form of pact or payment. Similar to giving reverence through incense or sacrifice of bread or wine. Just a lot more... personal.
As far as ideas of 'giving away' or 'losing' your soul, this is an odd misnomer by my opinion. I think it comes from the idea that soul is a 'thing' that you have inside of you. Like blood in your body, or oil in your car. Something that can be taken out or taken away. it is more accurate to say that soul -is- you. Not some part or piece. it is what comprises your fundamental nature and so soul and consciousness are permanently linked. You soul could not leave your body without taking the rest of you with it. A person can commit energy, emotion, focus, and will, and in the right conditions an entity can gain your energy through making you commit thought/attention, emotion, and focus... but they can not take your soul. The most they might be able to do is try and convince you that your soul is lost and/or you are somehow laid victim to them and their whims, in essence making you so believe that they own you that you don't resist or question and just go along with what they want. (IE become opressed/posessed.)
As an added brief note, jashinism is a fictionally devised religion for the anime Naruto. From what I understand after some brief research, it was (at least partly) inspired through an Indian religious cult called Thuggee. Which, in turn, was apparently a cult inspired by the following of Kali who is a Hindu goddess of destruction... among other things.
If you wanted to explore such a path, it would probably carry more value to go to the source material... as it were. IE getting to know Kali and the lore and doctrine surrounding her. ^_^
in most witches circles 'sacrifice' is merely means 'offering'. you are giving something, typically to a higher power [be it a God, a spirit, demon, dragon, faerie, whatever you work with] as thanks, a gift, or in exchange for something. long ago it meant sacrificing a person or animal [some groups still give animal sacrifices but it's rare] today we could give food or drink, or perhaps something we made or found we feel they would like. regarding blood, one can offer their blood, it is seen as very powerful so if you're offering your blood to something, i would get to know the being before giving your blood as it can become problematic for you down the road. [also, not all beings like blood, so research or speak with it before giving blood.] you can also use blood in spells to personalize it as well as increase the energy in the spell. you might also consider donating blood in honor of your deity/spirit/whoever you work with.
regarding selling or giving away your soul, that you cannot do. you could dedicate yourself to the deity/being you wish to work with, but you can't sell your soul for phenomenal cosmic power or whatever you want to sell it for. also jashinism isn't real either, it's based off of a dark form of Kali worship [see Indiana Jones for further information. it's not what happens obviously, but at least they're praying to Kali] very basic research can help you find the answer. [also before you ask, Death Notes aren't real either, though Shinigami are part of Japanese lore, no, Kyubee won't grant a wish and make you a magical girl, there are no Sayans, you cannot create a pokemon, and Future Diary is nothing more than a messed up anime]
