Can a person who was infected with toxoplasmosis during his/hers mothers pregnancy donate blood, or is it unsafe and thus rejected?
People who have had toxoplasmosis are not barred from donating blood (at least in the places I checked). While toxoplasmosis during pregnancy is dangerous for the embryo/fetus, if they don't have lasting damage from it, that person is just like any other person who once had toxoplasmosis, of which there are many (in some countries up to 80 percent of people are immune because they had toxoplasmosis at one time).
People with a currently active infection would probably be sent away, but since toxoplasmosis infection usually presents like a generic mild infection (muscle ache, mild temperature) and isn't tested for, that's no different than sending away someone with a cold.
When in doubt, ask the place where you are donating.
First, it'll be necessary to know if only the mother was infected during pregnancy, or if the parasite responsible for toxoplasmosis (Toxoplasma gondii) was really transmitted to the foetus. This was possibly done during pregnancy via amniocentesis.
If the person was infected in utero, then they are likely to carry the parasite, although they won't necessarily express symptoms. This can be verified by a specialized blood test (specific IgM and IgG antibodies will be searched).
Toxoplasmosis can be transmitted via blood donation. People who have been infected by toxoplasmosis are however eligible for blood donation, but they must give this information to the blood center as their blood will be specifically not given to people with immune deficiency (who could then be severely infected by the parasite).
These rules can vary from country to country. Be sure to check with the closest blood center.