It's generally recommended to apply oil to your black hair after blow drying, rather than before. This is because applying oil before blow drying can actually make your hair more susceptible to heat damage. The heat from the blow dryer can cause the oil to penetrate the hair shaft, which can lead to breakage and other types of damage.
Instead, it's best to wait until after you've finished blow-drying your hair to apply oil. This can help to seal in moisture and add shine to your hair. It's important to use a small amount of oil, as using too much can make your hair look greasy or weighed down.
To apply oil after blow drying, start with a small amount (a dime-sized amount is usually sufficient) and rub it between your palms to warm it up. Then, apply the oil evenly to your hair, starting at the ends and working your way up to the roots. Be sure to focus on the areas that tend to be drier or more prone to damage, such as the ends of your hair.