Blog commenting in niche blogs increases traffic to your blog site. You can comment on blogs or ask questions. Commenting may lead to conversation which leads to relationship between the blogger and commenter. You not only gain knowledge from the blogs but can become popular when you input relevant information.
Here are some sites providing blog commenting facility:-
a) adweek.com
b) Rebootauthentic.com
c) Exeideas.com
d) Allbloggingtips.com
e) Shoutmeloud.com
f) Johnchow.com
g) Webuildyourblog.com/blog
h) Pulkit.me
i) Bloggergo.com
j) Gradmoneymatters.com
k) bloggingwithoutablog.com/
l) howtospoter.com
m) quickonlinetips.com/
n) becomeablogger.com/
o) topofblogs.com