Describe the man who admitted Harker. Why didn't Harker knock at the door?

Description of the Count after he opened the door....

Within, stood a tall old man, clean shaven save for a long white moustache, and clad in black from head to foot, without a single speck of colour about him anywhere.

Harker didn't knock at the door because he couldn't find a bell or knocker. Thus, he waited.

Of bell or knocker there was no sign; through these frowning walls and dark window openings it was not likely that my voice could penetrate.