Blogs commonly have the capability for three different kinds of links. If the blog itself is reasonable quality, the best link to have would be an in-content do-follow link. That is going to have pretty much full value going to the target website. Links from the sidebar(s) are often pretty much like a footer link, there might be a lot of links but there isn't much value passed. Comment links are widely used for spamming and do not pass much value as far as I can tell, lots of volume is needed if you want value from it.
If you wanted to experiment, there are some fairly high PR academic type blogs that are highly moderated. If you are on topic with comments, you can judge results for yourself.
Backlinks from free blogs. By now Google knows which blogs are free, including the free blog platform they own. Focus more on really high quality content instead so that you can get natural links from places that are not abused by link spammers.
There is a ton of really high quality content on "free" blog hosting type sites. Sometimes they move to a paid type environment, sometimes they move from paid to free. I'm not one that would ever judge the content quality of a site based on the amount they pay for hosting.
To me, quality is providing the information or solutions users are looking for. The solution/information may be free or paid, but it fulfills a need. Tripod and Geocities changed the world, current free hosts just allow even more sharing of information for those who either cannot or do not want to pay hosting fees. I'll get off my soapbox now.
I have seen brands just basically buy/bribe bloggers to post reviews about there products in exchange for a dofollow keywordy link and it works.... They don't even really diversify the anchor text really on the links. They just make sure each review is unique and on a "mom website". Some brands I seen really just consist of 600 or so of these links bought on other website and not much else and manage to rank quite highly for competitive product keywords.
In fact these brands are the only things ranking now really in my niches that arent amazon and ebay after the last pandasssssss.