I'm thinking the "darkness" is there because you have the ability to detect light, so that when your eyes are closed they register "no light", which manifests as "darkness." A blind person's idea of what they see might be more like the view behind your head, it's just an invisible blank.
'Olfactory and gustatory changes are rather common in normal subjects and psychiatric patients; they can dominate the sessions of people with congenital blindness, who usually do not experience optical phenomena after the administration of LSD.' So, in people who are blind from birth, the perceptual changes will most affect the senses other than sight - smell, taste, touch and hearing in terms of both decreased and increased sensitivity during a trip."