Looking for a word attributing the state of revealing the past
Solution 1:
According to the example you provided,
the development of knowledge sheds light on the [word I am looking for] capacities of blood. For example, a blood test could determine who had syphilis.
I think the word you need should describe the hidden capabilities or the things that can be uncovered through blood, and not the word to convey the ability to reveal the past; but that's only my opinion. You're free to ignore this.
Maybe I can suggest a few adjective candidates for your query:
determining
revealing
hidden
disclosing