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.
Life's Solution builds a forceful case for the predictability of evolutionary outcomes, not in terms of genetic details but rather their broad phenotypic manifestations. The case rests on a remarkable compilation of examples of convergent evolution, in which two or more lineages have independently evolved similar structures and functions.
On a perfect planet such as might be acceptable to a physicist, one might predict that from its origin the diversity of life would grow exponentially until the carrying capacity, however defined, was reached. The fossil record on Earth, however, tells a very different story.