The answer to this question would require several assumptions.  Keeping someone in darkness for 10 years would require a rather sadistic psychology on the part of the captor and the likelihood of that being the only negative aspect of their incarceration is very low.  However, assuming that this person was being kept otherwise healthy, well fed, entertained (isolation would lead to severe psychological distress) and sheltered from heat or cold, the main effects would be on the pigmentation of their skin (skin color would lighten) and their ability to produce vitamin D which would lead to soft bones, possibly rickets.  The mammalian circadian rhythm is 26-27 hours long, so it would be possible to change the persons circadian rhythm if all other timed stimuli were removed or provided randomly.  Sleep patterns may be disrupted (outsize of the timing), with less sleep in sibling bouts (this happens in totally blind people).