When you state that humans can be categorised by one of THREE different blood groups, the blood group system to which you are referring - classifying groups as A, B or O - you are mistaken as there are actually FOUR groups in this classification system: A, B, AB and O.
However, this is only one of thirty classification systems in current usage as recognised by the International Society for Blood Transfusions (ISBT).
You might find this interesting reading:
In the "A.B.O." system, all blood belongs to one of FOUR major groups: A, B, AB, or O. But there are more than TWO HUNDRED minor blood groups that can complicate blood transfusions. These are known as rare blood types.
Whereas common blood types are expressed in a letter or two, with maybe a plus or a minus, a smaller number of people express their blood type in an extensive series of letters in addition to their 'AB-' type designation.
As for your belief in an alien race headed to Earth hell-bent on slaughter, I have to concur with your own opinion that many people will regard this ideas as "really out there" though I admire your conviction in your 'truth'.