well, it depends upon the blog owner if he/she has put nofollow tag in the blog comment than the link would automatically become no follow ..... you can't guess that all the blog commenting link building list is no follow !!
Certainly not.. Moreover, do not consider blogs which offer only do follow links. There are some popular blogs or websites who offer no-follow links, but you get some traffic to your website and it helps you to improve your website authority.
In most of the cases these days its is nofollow but not the case for every blog. To verify you need to check each individual blog by looking at the html or using some easy to use plugins.
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