The BA has a page on the Dark Side of the Moon.

He correctly points out that no side of the Moon is forever dark (in the sense of absence of light).

However the phrase "dark side of the Moon" does not actually refer to light at all. Time to consult with an unabridged dictionary:

Basically things which are unknown are "dark" in this context. Likewise consider the phrase: "in the dark." It is a noun this time, but referring someone not knowing something.

The far side of the Moon is hidden from us. Or at least it was hidden from us until space probes started taking pictures. Thus the far side of the Moon really is a "dark side." (Just don't commit an equivocation and assume that dark means absense of light.) Basically it is a case of confusing caused by an evolving language where words often have multiple meanings.

This issue came up in talk.origins recently and I just so happen to be looking at the misconception list and though I should bring this up.