Can I undercoat in black if I am going to paint white?

Yes, you can use a black undercoat if you are painting something white but ideally you should look at using a white primer.

If you have to use a black primer, you may need to give the white a few coats to achieve a pure white colour as the dark primer will affect the colour on top.

Bear in mind, the more coats that you add, you may loose some of the detail from your model.

That is why it would be best to use a white primer this will minimise the amount of coats required and give a better finish to your model.

That being said, using a black primer is a good foundation in certain cases where you do not want a perfect white. For instance if painting a weather building or some skeletons as the black primer will help create the shadows.