Question

why does our hair looks black after applying oil?

Answer 1

Similarly, why do things look darker when you spill water on them? Surely putting a clear substance on something doesn't change its color, so why does it look darker?

In both cases what's happening is light reflects between the layer of hair and surface of the oil causing it to be absorbed more and reflect less. Light will also bend by an angle in the oil or water causing it to go in another direction rather than towards our eye like it might normally do.