Evolution implies a change for the better, doesn't it?
Not necessarily. The textbook definition of Biological Evolution is change in allele frequencies over time, which, by the way, is an observable fact.
That means, somewhat simplified, that if 10% of a population has blue eyes at a certain point in time and ten years later that number has changed to 11%, that evolution has taken place.
Also, "change for the better" is a tentative term. In short, evolution does not mean a constant uphill struggle for more complexity but rather that natural selection works on the overall adaptation of a species with regards to its environment. And if less complex works better than more complex, less complex is often the direction it takes. This can be seen in certain cave dwelling animals that have degenerate non-functioning eyes as having eyes in an environment without light is a waste of resources.