I would like to know if it's possible to add an erratum for my own book just to mention/reference a blog post from another author.
I just forgot to add a reference and I think it's important to add a reference to him as soon as possible.
Writing Stack Exchange is a question and answer site for the craft of professional writing, including fiction, non-fiction, technical, scholarly, and commercial writing. It only takes a minute to sign up.Sign up to join this community
If your book is published by a third party publisher, this would be a question for them and they would be best placed to explain their procedure. There may be costs associated with the addition, which they will probably try to pass on to you (it's not unreasonable for them to do that).
If the book is self published and you have immediate access to the version being distributed, this may be a change you could make yourself - again, this will depend on any third parties involved in publication or distribution.
Apologies for an "it depends" answer, but if other people have been involved in publication and distribution they will have to be considered.