What can dried blood tel an expert?

It depends on which experts we are looking at,

If it’s for a crime scene, then the person can find evidence, for example to whom does it belong, the victim, the murderer or someone else

If it’s for medical reasons then it can identify your blood type, the iron levels...etc

The answer is joseph is observing the colour of the reaction mixture to see whether proteins are present in the given solution.

Idont see a question here