Have you tried clearing your browser?
windows – crtl F5
macs – apple r
Does the blog in question (for which you have not provided the url so I cannot check it myself) have a contact page or an email address posted? If so then you could use one or the other.
Please provide the following information:
(1) the url for the blog to which you are referring
(2) what your operating system is
(3) what browser and version of it you are using
(4) were you previously able to comment on the same blog from the same computer using the same browser?