Brown to natural hair

I dyed my natural black hair to a chocolate brown/auburn colour in summer last year. I managed to dye it without bleaching my hair as I used the Nice and Easy 11 A Ash blonde ultralight colour. I think the colour either stripped away the pigment or, deposited a lighter colour making my hair appear brown?

The brown colour has grown out and faded a lot now and I don’t like it so I want to dye the brown hair to a black shade - maybe a tad bit lighter than my natural black. My hair is also not jet black and has specks of very dark brown in the sun. I do not want to accidentally dye my hair darker than my natural hair colour.

The only alternative would be touching up my roots however when I originally dyed it, a little section at the front of my hair had had roots because I accidentally left it for a couple of min longer.

Because my box dyed hair is unbleached does that mean I have to buy a jet black hair dye to balance out the fact that the hair has been lightened. Can you deposit a darker colour onto unbleached permanent dye or will I need to bleach it to go black? Does brown + jet black dye = natural black?

How can I do this at home as I don’t have enough to spend on a professional salon. Can I use box dye on the old colour?