Check your correctly fitted bra to make sure it really does still fit. If you had it fitted recently, you can skip this step. Otherwise, remember that your bust size changes frequently over the course of your life, and your black dress will look its best when your bust is well supported.
Look for a V-neck that doesn't go so low your cleavage looks like the Grand Canyon. V-necks are slimming, and draw attention away from your large bust as long as the dip isn't too revealing. A sleeveless V-neck gets bonus points, because your exposed shoulders and arms also draw attention away from your bust.
Avoid turtlenecks and other high-necked dresses like the plague. If you wear a black dress like that, you might as well paint targets over your boobs and hold a big flashing arrow sign that points at them.
Steer clear of any black dress with busy patterns or decorations around the bustline. 3D black roses are beautiful and striking, but have no business drawing attention to a part of your body that already draws enough attention to itself simply by being there.
Find other parts of your body to emphasize, so that your body looks balanced. If you have a tiny waist, choose a fitted dress that emphasizes it. Long legs practically beg for a nice, long, flowing maxi-dress to bring out the Amazonian goddess of your stature.
A correctly fitted bra should be snug and supportive around your ribcage, but not painful or restrictive, especially when you move. Your boobs should fit snugly inside the cups, with no spillover or puckering of the material. If your bra has straps, they should comfortably rest on your shoulders -- not ever dig into them hard enough to leave marks. All the weight of your boobs, when properly supported, should be supported by the band around your ribcage. Underwires should support comfortably, not poke or cut into you.
Wear any other undergarments you plan to wear when you wear your dress as you're trying dresses on. If you already have shoes for the dress, bring those along too. Different undergarments and shoes make a huge difference in how a dress looks on you. They might even drastically sway your feelings on whether a particular black dress should be coveted and adored or trashed and abhorred.
Shop at clothing stores that cater to ladies with larger busts. You may not always find exactly what you want, but it might give you a better idea of what you're looking for. Narrowing your choices down even a little bit is always helpful when trying to decide on something.