The problem with denim in any colour is that it fades rapidly. What may be a nice, solid black pair of trousers at the beginning of term, will have faded patches and grey areas by the end of term, making them into 'jeans'. The best fabric to get would be a heavy duty black cotton drill, available at most good haberdashers. It washes well, is soft and breathes, but retains its colour and doesn't rub too easily. Being cotton it's easy to sew as well!

Good luck.

LTS