Even without the thinning hair, I would suspect it is the early signs of a condition called seasonal flank alopecia (and I think that's where Jan was going when asking if he was thinning
).
It it's the case, you may well find that the darkened skin disappears over the summer, but returns toward the end of next winter - and may involve some hair loss in the same area.
SFA, as the name suggests, is seasonal hair loss - almost always concentrated symetrically on the flanks (though it may also spread elsewhere on the torso). It is thought to be caused by the limited exposure that many dogs get to sunlight over the winter months - and limited time in the dark, since we spend so much of our lives under artificial light. Anyway - reasonable time in natural sunlight is required to trigger production of the hormone melatonin, which amongst other things is linked to hair growth. Whilst sunlight is the trigger, melatonin is actually only produced during the hours of darkness. So one can see why limited exposure to either of those can result in inadequate levels of that hormone being produced.
Some dogs respond well to their owners ensuring that they get adequate time in daylight over the winter (not easy if you work all day), whilst some others respond well to melatonin supplementation. Some just don't seem greatly helped. And in any case, the hair generally grows back - and the darkened skin disappears - when the days grow longer again during summer.
Not sure how clear these pictures are - but my dog can be an example. In this photo, you can see the darkened patches on his flanks (actually 3 large patches - obvious even if he's brindle):
http://www.boxerworld.com/photo/showphoto.php?photo=75843
Does that look anything like what you're seeing? If it does, then search the gallery for some more pictures of seasonal alopecia - there are several cases you can see.
Fist sized - it certainly is not freckles
