Can you detect human ABO-Rh blood type from a saliva sample?

I don't think you can. Testing blood types requires the blood itself to test and see what antigens the RBCs has. From there we can conclude which blood type you are
A+ or A-
B+ or B-
AB+ or AB-
O+ or O-

Upon further research, it seems like it's possible, but hard to do.

In a blood sample, red blood cells have the characteristic type A,B,Rh antigens on their surface. You mix a small drop of blood with separate wells that are treated with A/B/Rh antibody. If the antigen is present, the antibody will react, and the red blood cells will clump, and due to their large size and color, the reaction will be visible.

Approximately 80% of the population is known as a "secreter" that secretes these blood group antigens in their saliva. So the same A/B/Rh antibody reageants would produce a similar reaction in a saliva sample. From what I understand, the difference is that you can't see the reaction since the antigens aren't attached to large red blood cells. A more complex ELISA test should work, but I can't justify that complexity over using the easier blood sample. Also, the 20% of the population that are non-secreters don't have blood type antigens in their saliva in significant amounts so a saliva based test wouldn't work.