Spaniards call this "Pacto de Sangre." It sounds morbid and makes Filipinos of old look like blood thirsty natives. Maybe our ancestors were vampires, but of course they're not. Don't think they want to taste the blood of men from a different race. For them, blood is sacred.
Blood is life. To sign an agreement with blood is to sign with one's life. It becomes a bond to protect and honor the agreement with one's life. To enter into a blood compact is to enter to a lifetime oath. The bad thing about this is, if one dies, the deal is off.
Other cultures have seriously misunderstood this practice. It is not drawing blood and drinking it straight up. At least we use glasses for it, while the vampires suck from the body directly. The blood compact is not barbaric since they mix it in wine and its only a couple of drops of blood. Seriously, what would a couple of drops of blood do to you in an alcoholic drink?
On the latter part of the colonial history of Spain, almost the same thing happened with the Katipuneros. Signing also with blood. But of course since there were thousands of Katipuneros, it will be like a blood party. That would be a taboo. Instead, these men signed their membership forms with their own blood as ink. Very creative, really.
We lost our independence through a pact of blood. We gained it back through a pact of blood. Blood is really important for us Filipinos. Blood and sweat.
"Dugo at pawis ipinuhunan ko dyan." It means "I toiled and sacrificed myself for it" and when we Pinoys say that, it means we are not going to let things go easily.