Just go to any forums/blogs and view the source to see if the links are dofollow. If they are have your way with them. But we all know that comment links are lowest of low when it comes to "SEO juice" so it might not be worth your while.
Before asking for blog and forums, specify the industry you are targeting with your website. Then it would be easy for others to suggest blog and forums for you.
Blogs and Forums both are useful methods in order to gain some handy links for your site. You just need to work in ethical manner without spoiling your image. In forums : Give your active contribution by posting useful things. In blogs : Make helpful comments towards the topic Make sure about two things : You should have dofollow and high PR sites in both methods.
Most Blogs provide Do follow links but forum posting sites have turned their way to linking back. They're now providing No follow back links and even bad they show signatures when you're signed in. Unregistered users and bots cant see these links.
Tip: Check the PR=Page Rank on these blogs/forums. Hopefully its around your niche or industry. You can simply type in google "internet marketing forums" and you`ll see digital point is PR 6. Anything from 1-10 is high. I hope this helps your question. If you have any additional specific questions, feel free to message me or reply by quote.
Yes, blog and forum through you can get backlinks. When you do comment on blog and forum make sure those forum and blog have do follow links and also high PR. Those site's will definitely give you good backlinks and as well as good traffic.
There are no. of blogs which offer guest blogging (where we can easily get backlinks to our websites or blogs). I usually find such blogs in the Technorati Blog Directory. And while coming to the forums, i don't intended to use forums for just backlinks.
You can get a useful firefox plugin for free NoDoFollow that will allow you to check the Blogs and Forums you want to post on to confirm if they are DOFOLLOW. This is one of the many useful tools I use in my artillery.
Here are some of my recommendations: web 2.0 sites: Squidoo, hubpages, forums: digital point, warrior forum, site point Pretty much anything on top 1-10 of googles page. You can utilize "Site Similar Search" to make it easier. - this basically helps you find it thinks is similar to yours based on keywords and content If this helped you, I appreciate rep points. "Click the thumbs up icon"