after browning a roast, the remaing oil looks dark. what should do?

Maybe too late for this roast. But useful for futures.
If the darkness comes from bits of the roast, you could use the oil for gravy.
If the darkness comes from several uses of the oil, strain it and check the smell. If smell is ok, store in fridge for up to : months and use again. If smell not ok, discard - this oil has served its purpose.