How to ombre dye?

Using bleach to 'lighten' a black dress is risky as the bleach might eat into the fibres, completely weakening them. Even if you use fabric-guard bleach, you will need to dry the garment single layer and completely flat in order not to get any streaked or blotchy dyed areas, due to the water dripping down.

Black can be very fashionable summer colour if it is highlighted by a white or contrasting light colour, such as pink, yellow, light green. I would suggest either sewing some trim on the dress, or wearing it with big white jewellery.

i totally would not use bleach.use fabric paint and make a design

i agree with Flipturn. Not likely that your plan will turn out the way you are imagining. The black can be very sophisticated, Just accessorize with jewelry, light-weight sweater, or shoes in white or bright colors you love. Maybe wear a bit more makeup/lipstick so you don't feel too somber in the dress.