"to get the attention of something" ― I think this is not about "getting the attention" of "something" out there. I think those who use or/and offer blood, knows already who they are reaching out to and what are their personal intent. It is usually not to seek attention - they likely got that already -, but to show their devotion to the matter, to give a valuable return to the entity who is willing to work with them, to pay a proper respect or to appease the entity.
Blood is charged with powerful energies, as it is practically a life essence that contains the very "programming" of your physical existence - therefore, it also forms a link between your body you possess momentarily and the entity to which you offer your blood -. It is a fuel, not only for your flesh and bone shell that makes it possible for you to function on the material world as you are, but for multiple practices both energetically, spiritually and even physically.
As a potent fuel, it can empower your spells, your rituals, the desired object you wish to charge etc. however, we all have our principles when things comes to blood offerings.
Some people make blood offerings to almost every entity they're working with, because for them, this feels just right. Because this is the way how they pay their respect and gratitude with such an indeed precious gift. While others - like me -, only use blood offerings if the entity in question has a close bond with them and if they already have a lot of experience, time spent, and trust built up.
It doesn't mean that these people are not willing to use blood for work, but as an offering it has a different purpose.
However, I'm quite squeamish when things comes to blood extraction.
Pricking my finger? Hell no. I prefer cuts. Take it or leave it.