Dyeing a Black T-shirt?

I redye my black jeans all the time when they get faded. i use the ritz die the liquid one works best. you can use the powder one as well. which ever one you pick though run your washer full of the hottest water, gently put in either dye solution by pour it low into the water basin right onto the water, let it agitate first for a second then add your garment. the more garment you have the less color result you get in your fabric the and the less fabric garment you put in the more darker it will appear.

also choose which water level you prefer for darker results. let it set in the water bath for awhile then run through the cycle as usual.

Remember after you are done with your dying to fill your washer again with hot water a bit of laundry detergent and a bit of clorox and run a regular cycle so your washing machine walls will clean them selves of the dye left on.
as far as the writing on the t-shirt, I think it would just cover it up slightly with dye not totally be gone. hope that helped