That said, lots of people don't even know their blood types and that shouldn't be used as a barrier when the DB does need more ethnic diversity, period.
I understand the need for diversity in the database and you can argue that not filtering by blood type would expand the diversity, these appeals almost always ask for people of a particular race or heredity which limits diversity and seems to conflict with that notion of increasing diversity.
A compatible donor has to have a compatible blood type and matching leukocyte antigens. The later is expensive to test for but race can be used to exclude a large segment of incompatible donors. Blood type is harder to check obviously but compared to leukocyte antigen compatibility test it's cheap an easy. We're talking less than $10 vs $100+ for a leukocyte antigen test.
These are personal appeals and the two factors that can be used to cheaply and quickly narrow the field of potential donors the most are race and blood type compatibility.
That said, lots of people don't even know their blood types and that shouldn't be used as a barrier when the DB does need more ethnic diversity, period.