My interpretation of the it is that 'Blood' is for family, meaning that they will always be there for their families, and 'Sand' because maybe they feel the ocean is there true home.
That is just my opinion though...others might feel differently!
Nicely put @LauraPhilly
Yep they come from broken homes and it seems by the way they acted when Stu got his tattoo that once you get it - you are officially a RB and apart of their family unit. It represents they stick together, look out/after each other and they are also bounded together by their love of surfing.
--good thing there moto isn't 'Blood and Drugs'
Thanks!