That shouldn't be a problem.
I'd just say "you're lucky most of us have to work at it". Frankly, as your title suggests, making such a remark means you've no racial stereotypes and take people as you find them. I would think it weird if someone with natural dark skin were offended that you didn't realise their "ethnicity".
(Me, I'm not... in any way intoxicated, Fri 17 Aug 2012, 14:27, 1 reply)

That was my take on it...
..but what really burns me is that I so nearly said, "sorry" - which could be taken in two ways:

a) Sorry for not realising your ethnicity - perfectly acceptable.
b) Sorry you are ethnic - very very bad.

T.b.h. I don't think she was miffed that I hadn't noticed she was 'mixed race'; but I found the whole thing quite cringe-worthy.
(minimalist ..., Fri 17 Aug 2012, 14:59, closed)

Yeah, "Sorry." could certainly be very awkward.
It is a weird situation though so I can understand it feeling a bit tense.
(Me, I'm not... in any way intoxicated, Fri 17 Aug 2012, 16:51, closed)