There’s nothing unusual or unique about the format for citing online sources such as blogs or forum posts:
Author, A. A. (Year, Month Day). Title [Description]. Retrieved from URL.
Bauer, A. (2017, August 21). My view of the eclipse [Blog post]. Retrieved from https://www.andysscienceblog.com/eclipse
Notice that you don’t have to give the name of the blog itself.
You can cite a comment on a post as follows:
Slinky. (2017, August 21). Re: My view of the eclipse [Blog post comment]. Retrieved from https://www.andysscienceblog.com/eclipse
If the author of the comment has used a screen name (as here), cite that.
Styledog. (2011, December 5). How to cite videos in APA [Video file]. Retrieved from https://www.youtube.com/watch?v=dCXdpfE7y8Y
For more information, see our page on audio visual sources.
Cassidy, B. (2017, December 5). Re: Psychology of train robbers [Online forum comment]. Retrieved from https://wwww.redditio.com/robberyforum/trains_comments
For more information, please consult pp. 214-15 of the Publication Manual of the American Psychological Association (6th ed.).