Why car radiator is design with thin fins and black in colour?

Car radiators have fins to allow the flow of air to enter, but as well as having a bit of protection. If it didn't, you would have a car with a hole in the front.

Also, they are black to absorb and radiate heat. Black bodies radiate heat better so that is why they are black. The thin fins improve the overall surface area so that heat is dissipitated to the ambient efficiently. Hope this helps.