<a href="http://example.com" title="my keyword"></a>
Can I build back links like this?
Is this black hat or white hat?
As you surely have already understood, this is not a good way of building backlinks. That's why I would say it is part of blackhat techniques.
A little advice: when you build a backlink, think about users first. Only put useful links for users and don't try to trick your SEO because otherwise, you will most probably get a Google penalty someday.
Google would consider a link with no anchor text to be a "hidden link". Google has a hidden text and links page in their webmaster guidelines that calls out a slightly less egregious case as black hat:
Hiding a link by only linking one small character—for example, a hyphen in the middle of a paragraph