The most simple black hat SEO techniques are - encrypt words in images with the help of banner codes or something similar because while crawling google bots don't crawl through images. Insert words which mix with the background color .... This is even more simple just put a text editor in between and enter high density words which contrast with the background color so that they don't even seem to be present .....

But these are quite risky because if they are not performed with utmost care and google detects them, then you sure are gonna get banned for it