Which is the best place to post a URL to get a backlink?
- Forum profile
- Answering a question and giving a related post URL as the answer
Do Google's algorithms consider all of these to be spam/blackhat?
If you can provide great value to the community/forum, then please go ahead and help with answers. A link or two once in a while will be considered acceptable, but you do need to establish credibility before you start posting links.
StackExchange is a great example of such a forum :-), do you see people spamming links left right and center here? No.
Do you see folks helping out and adding value to the discussions? Yes.
Any form of user generated backlinks you should step carefully, Google does not want webmasters creating links as this is called manipulate search results. Google loves naturally made back links by the visitors of the site and not the webmasters.
Google over recent years has nerfed links from user generated sites such as:
And so on, the list continues to grow, this is because again Google does not want users creating their own links to manipulate search results. Webmasters have been using tools like Scrapbox, Senuke, Sicksubbmiter, GSA and more to make links in the 10's of thousands at a time.
Also when I say nerfed, I mean they have been nuked! and hold little to no value at all... and enough of these links with the right anchor text your find your site ranking very badly for those keywords. Stick to creating great content, use paid traffic like Google Adwords if required (I know its expensive but its viable for a long term strat and good for new sites.)
The most important thing in link building is to make it as much natural as it is possible. Just as if it has been your potential client doing so and from this perspective link in forum signatures have a really low value to Google.