Hello Boinng I know that it is several years later after your post however I thought that I could help you out some. The actual list of the Blood Island Series is:
BRIDES OF BLOOD
MAD DOCTOR OF BLOOD ISLAND
BEAST OF BLOOD
The reason why Terror Is A Man is sometimes included is because of two reasons:
It was made in the Philippines (like the blood island series), but the bigger reason is that later in life it was AKA Blood Creature / Creature From Blood Island. However these titles were done later when the movie was re-issued. If you ever see the movie actually it is a remake of The Island of Dr. Moreau.
Now the other movies you mentioned are considered part of the series for several reasons and I hope this makes sense:
Beast of the Yellow Night - John Ashley starred, filmed in Philippines
Twilight People - John Ashley again, Philippines again, same kind of plot as Terror is a Man and the big reason for these two is that they were done by the same director Eddie Romero, who did all of the movies.
Now for the other ones you mentioned Blood of the Vampires, Horror of the Blood Monsters etc is because when they were released on DVD several years back they were packaged together in a box set called THE BLOOD COLLECTION. Whey this was done I don't know but that is why so many people think they belong.
If you are just looking for the Blood Island series it is just the top three movies. I hope this helps
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