In _The Voyage of the Beagle_ Darwin condemns the treatment of slaves
in Chile, arguing that they are people just like anyone else. He
specifically speaks against racism.
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Gould writes of Darwin's relationship with Fitzroy, the captain of the
_Beagle_, in "Darwin's Sea Change" in _Ever Since Darwin_ (page 32):

Darwin and Fitzroy maintained a tense relationship at best. Only the
severe constraints of gentlemanly cordiality and pre-Victorian
suppresiion of emotion kept the two men on decent terms with each
other. Fitzroy was a martinet and an ardent Tory. Darwin was an
equally committed Whig. Darwin scrupulously avoided any discussion
with Fitzroy of the great Reform Bill then pending in Parliament. But
slavery brought them into open conflict. One evening, Fitzroy told
Darwin that he had witnessed proof of slavery's benevolence. One of
Brazil's largest slaveholders had assembled his captives and asked
them whether they wished to be freed. Unanimously, they had responded
"no." When Darwin had the temerity to wonder what a response made in
the owner's presence was worth, Fitzroy exploded and informed Darwin
that anyone who doubted his word was not fit to eat with him. Darwin
moved out and joined the mates, but Fitzroy backed down and sent a
formal apology a few days later. [End of extract]

So Darwin was certainly no lapdog to, what would today be considered,
racist attitudes towards slavery. It seems likely that he was an
opponent of slavery. (Anyone familiar with his autobiography or other
writings that could confirm or refute this?) He probably had typical
ideas of his day about Europeans being superior to Asians, Africans
and New World natives, but he was probably, by our standards,
relatively enlightened. (Of course, this is largely conjecture on my
part. Anyone got something more concrete to support or undermine this?)