50 volume developer, which they do still make, we sell it at Sally's Beauty Supply is usually only available to professionals for purchase, it is used in bleaching, highlights, whiting, it is extremely harmful to the hair and should only be used by an experienced hair color specialist, even then a lot of stylist don't even want to use it because if the damage it does.
As a retired hairdresser I've never seen 50vol peroxide. It could be out there, I haven't been looking for the last 10 years. 40 and 30 volumes are used for faster and higher lift. Higher volume peroxide can bring more damage, so keep a good conditioner handy and use a leave-in moisturizer each shampoo time thereafter. If you are bleaching hair near the root you would use a "hair lightener/bleach" with the peroxide which does not burn the scalp. If you are bleaching away from the scalp, like frosting or streaks, use Frosting bleach mixed with peroxide. If your hair is black and you only want to make it a "brown", you can do a test with some "tints" and start with 20vol. Or whatever agent the product recommends. Usually anyhone with naturally black hair does not look good in another color. Lightly sprinkled highlights are gorgeous through black hair, but they would be golds or browns or silver. If your hair is dyed black, you need to get a "color remover" and after using that you can decide what to do next depending on the color you have left.
They have not made 50vol peroxide for years as it can make your hair fall out and break off. Please throw it away. To lighten your hair you will need some bleach and a 20 vol peroxide. This will lighten your hair by 2-3 shades but be careful. Watch your hair as the roots will lighten much quicker than the ends if your hair is coloured black. You may need to do it more than once but give it a day or so before you do it again. Treat it with a good conditioner as bleach drys out the hair and can damage it. When you are satisfied with the lighter colour, put the new colour on and that will even everything out. Or see a hairdresser.