Thou knowest the mask of night is on my face,Else would a maiden blush bepaint my cheek.She says she is wearing a mask, because he cannot see her face in the darkness, which covers her face like a mask. This is interesting considering that when they first met, Romeo was wearing the mask.Anyway she says that if she wasn't wearing the mask of night, a maiden blush would paint her cheek. This does not mean that she would be applying makeup and adding a little blush to her cheeks. It's not literal. It means that her cheeks would be coloured as if they were painted, if she wasn't invisible to him.