I dont think an alien will be a blob. If aliens are out there they should have evolved just like us. They should have eyes and be walking on two legs. In short if there is any life out there then it is likely to be very similar to us.
The underlying reason for convergence seems to be that all organisms are under constant scrutiny of natural selection and are also subject to the constraints of the physical and chemical factors that severely limit the action of all inhabitants of the biosphere. Put simply, convergence shows that in a real world not all things are possible.
It seldom seems to strike the ultra-Darwinists that theology might have its own richness and subtleties, and might strange thought actually tell us things about the world that are not only to our real advantage, but will never be revealed by science.