As long as the subdomain can be entered through the main site it will get a ranking in Google and it will not be considered as spammy. So, when using a subdomain, be sure that it is backed up by good links from the main site and that it can be crawled by the search engine spiders.
If you are just using it as a stand alone landing page full of keyword rich content, this will be considered spammy. (i.e. using it as a landing page for links on other sites.)
This is only allowed if you are using it as a landing page for Adsense and those pages will normally not be shown in normal SERP.
In short, subdomains are not considered to be black hat if they can be spidered from the main site by the SE spider as it will be regarded as regular site content.
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