a dark, young person came —

 one who has black complexion (in kali yugam) —

emperumān who is of dark complexion —

(contrasting the mattress-s complexion) having black form —

emperumān who is black in complexion —

emperumān who has a thousand divine names, has reddish, divine eyes and who has a dark divine form —

krishṇa, who is dark in complexion —

being the one with dark complexion —