Does a normal PTT blood test mean low risk for developing blood clots?

No. You should draw no conclusions about tendency to develop clots from ANY of the ordinary hematologic parameters.
It is a good sign. Partial thromboplastin time (PTT) is a blood test that looks at how long it takes for blood to clot. A normal PTT is reassuring and an indication that a person does not have certain conditions that might cause prolonged bleeding eg. hemophilia. The PTT test looks at only some of the proteins or factors involved in the clotting process, and is more meaningful if done along with other blood tests.