I've periodically had problems leaving comments on Blogspot Blogs. When using Internet Explorer I could only leave a comment if the blog had a pop-up comment box.
I began using Google Chrome and that worked fine until I started receiving Malware Warnings on my blog which was a problem that Google Chrome "found" while trawling blog sites.
It turned out not to be a problem at all because the link on one of my gadgets was a counter and not malware. In fact it had been there since 2009! I deleted the gadget, but in the process of trying to find the problem I had changed some settings. One of which was blocking third party cookies (Under Settings, Advanced Settings, Privacy, Content Settings). The comments I have left on blogs since then had not shown up but as many blogs review comments before allowing them I hadn't noticed.
I removed the check mark on blocking third party cookies and can now leave comments.
By the way if you receive a Warning Malware message on your blog, go to the Google Product Forums and leave a message to be forwarded to the Webmaster, they will respond usually within 24 hours with the exact location of the "problem".