Statisticaly u need 34 pulls to have a even chance of getting 1 black unit. I did 28 and got none until now (and two plats, statistics say 2.8 plats). So if u roll twice and get a black theres no reason to complain about it even if u have gotten that unit 3 times already :'P
Also dont give too much about chance up. I pulled 9 golds. Not one of them was Stray. (couldve needed him for AW spica/rita)