Now days many bloggers are using nofollow attribute in blog commenting section, so if you get nofollow link through blog comment it isn't useful in increase of Page Rank of your website. So, i agree with "omalab". Blog commenting is only worthy if your comment is informative enough to attract user and blog post is relevant to your business.
If you exchange link with your niche website of high PR, i don't think that Google will have any problem in it. Also guest blogging exchange is not considered as link exchange, we can say it as "content exchange" or "guest post swapping."