I use Black Cumin seeds, fine for savouries and breads, they look the same although taste a little bitter which I don't mind as they are full of health benefits too.
http://www.livestrong.com/article/338966-black-cumin-seed-benefits/
I don't think there would be anything alternative for poppy seeds taste - wise. If you only use it for sprinkling over the bread, I would substitute it with sesame seeds. But if you use it as an ingredient, than some people claim peanut or almond paste can work. IMHO, I would leave it out completely/