nodraw doesnt cast shadows, and the blocklight texture should cover a doorway or something you dont want light to pass through. There is another texture like this that does almost the same thing called block_los which i believe has something to do with face styles.
It was used in HL:S on the room wall where the gman and a scientist were talking at the beggining. Its covering the windows and I think it blocks lights because its creating a strange looking shadow, but it must do more than that.