I don't think it is a Black Sumatra....face would be dark altogether at this age, and the body type is wrong...she looks more layer than jungle/game with the dove type head you see in Sumatras.
I think it is likely either a Jersey Giant or Black Australorp. Might still be a Black Sexlink if...
x2 we need a better photo preferably with the bird individualized in the forefront.
Black feathers and black legs cover a lot of different breeds...plus age of bird would help....some start black but get subsequent colors.
Your black bird with black legs could be an Australorp or Black Sex...