Question about forum links and blog comments...

OK, first of all, I know that forum posts and blog comments en masse from rando sites is basically junk, and more often than not, detrimental to your SEO success.

BUT, I had a couple quick questions in regards to this:

First of all, if you post a link from a RELEVANT forum, where you have a decent amount of posts (not that that matters, but it makes it so that the link is FAR less likely to be deleted) and you post it with some genuine content on your subject matter (a link to your blog, for instance) does this still count as a good link?

Not count at all?
Can it still hurt your rankings or put you at greater risk of being sandboxed?

And in regards to blog comments... let's say you're trying to rank for the keyword "how to ride an elephant" and you pop that into Google, and one of the top 3 results is a blog post on that very subject which allows comments.
If you then make a genuine comment on the article, and then, say, link to a blog post of your own and say to people that that has more information there... does this still count as a genuinely good link (because it's relevant)? Would it still possibly hurt your ranking and/or put you at a greater risk for sandboxing?

Would appreciate some advice on this.

Thanks in advance,
-Cam