SUMMARY: Some merles get darker.
I like blue merles when they have lots of nice, light gray.
Boost has been getting darker over time--compare, for example, the left ear:
In fact, Boost had a white spot on her nose as a puppy that I kept thinking was a flower petal and kept trying to brush off. That white spot has completely vanished.
But here's a really interesting extreme example of darkening.
(Thanks for the post idea, BrisbeeTheWhite aka Elite Forces of Fuzzy Destruction.)