By chance, I found out that my blog had been blacklisted by Quttera. No big deal, because it happens from time-to-time due to the nature of the content on the site. But I discovered that it isn't just my blog, but Quttera also block industry-leading sites such as Cisco, VMWare, Sophos, MITRE, AVG and Phishtank.
For example, at the time of writing the following domains are all blacklisted by Quttera (clicking the link shows the current blacklisting status):
Cisco's blacklisting entry looks like this:
raising a ticket but the most prominent link leads to a paid service for £60/year. Hmmm.
I guess there are many, many more domains that are in a similar situation. Perhaps you might want to check your own web properties and share your findings in the comments?