Human skin gives off an array of gaseous substances, including carbon dioxide and lactic acid, both of which attract mosquitoes  However, neither of these two substances, whether alone or combined with one another, will attract mosquitoes as much as a bare human arm will, even in complete darkness, where a mosquito has no visual cues
Ttherefore, some other gaseous substance given off by human skin also attracts mosquitoes
Mosquitoes are no more successful in finding a bare human arm in darkness than in light.