LOL you don't have to kill followers to fill the star! It's not the Ebony Blade! hahahaha
I went with Black Star this time around, because I do kill more humanoids (bandits, rogue mages etc) and so I get more grand souls that way. There are not many grand soul creatures around! So yea, black star is definitely the way to go if you need grand souls!
Yes, you have plenty of enemies to kill, Riften is nice as you can hunt in the ratway sever or just trap the random thieves who are killed by the guards at night.
At low level your only source of white souls is marmots who are rarer and harder to kill.
However if you don't want to raise enchanting and make self enchanted gear the white star work just as well.
I would also feel a bit dirty as a Dumer double crossing Azura.