This comment also makes no sense... they said completely black out a room, which blinds alone cannot do.

Yes they can, the thing you're looking for is literally called black-out blinds. Installed properly, they don't let any light in even when it's high noon outside.

I was not referring to specialized black-out blinds, just regular blinds.

Blinds bolted on to the window frame can't, but blinds integrated into the window frame definitely can.