Blood Island movie list?

There are 4 films in total.

1. Terror Is a Man (1959) Dir. Gerardo de Leon
2. Brides of Blood (1965) Dir. Gerardo de Leon, Eddie Romero
3. Mad Doctor of Blood Island (1968) Dir. Gerardo de Leon, Eddie Romero
4. Beast of Blood (1970) Dir. Eddie Romero

The film "Brain of Blood" (1972, Dir. Al Adamson) is sometimes listed among
them but doesn't really belong to them.
Also, this hardly can be called a "series", as these are just four films
loosely based on each other and featuring a monster roaming an island.

I get confused myself, but I think it goes like this. "Brides of Blood" (1968) , "Mad Doctor of Blood Island" (1969), "Beast of Blood" (1970) and "Brides of Blood" (1971) all released by Hemisphere Pictures.
Hope this will help you.

Not sure why there's such confusion here. I own and have watched all the movies discussed here. The definitive "Blood Island" trilogy is:

1. Brides of Blood (1968)
2. Mad Doctor of Blood Island (1968)
3. Beast of Blood (1971)

All three star John Ashley, take place on the mythical "Blood Island," and concern mysterious goings-on with mad doctors and their mutant creations. The first movie is not connected to the other two plot- or character-wise, and John Ashley's character has a different name, but the third is a direct sequel to the second, literally starting where Mad Doctor left off. They all share a similar look and tone, feature mild nudity and gore, and were filmed in color.

Terror Is a Man is a fine movie, generally regarded as the first Filipino horror film, but is unrelated to the Blood Island movies other than being directed by Gerry DeLeon (who co-directed Brides of Blood and Mad Doctor of Blood Island) and taking place on an island. It is actually a loose remake of Island of Lost Souls.

Brain of Blood is completely unrelated to all of these, its only loose connection being that it was partially filmed in the Philippines and released by Independent-International, who released the Blood Island films as well.