Dont know the "official" name for it, but they are window caps. Blazer doors are just truck doors that have the top part cut off. When they cut the door from being a Truck door, they put those on the windows from the factory.
Looked through a few Venders for you, and didnt see any...I even looked through Goodmark
Might want to go to the parts section, and post a "wtb" post. I am sure that someone like Teeitup who has a bunch of trucks, might be able to get a set for you.