What is doorway page? are they come under black hat SEO?

Doorway page is fake page that user will never see. It is for search engine spiders. They are come under black hat SEO.

Doorway pages are against the Google guidelines and can still work for results but lead to a de-indexing of the site if caught, in which case you would have to fill out a Reinclusion request with G.

Doorway pages are typically large sets of poor-quality pages where each page is optimized for a specific keyword or phrase.
Since both are intended to deceive search engines, they are termed as black-hat.

This is purely black hat technique, which works against the search engine guidlines...so if we follow this technique we can't get results to your website or blog..so better to avoid this kind of fraud techniques to getout from spam thing by Google..