Content of killed.html shown as text

[JJoe]

I'll guess, the blocked address does not appear to be an image and that the Proxomitron returns "killed.html", Content-type: text/html.
However, the browser is expecting a file for a dynamic element or an image and displays the contents of "killed.html".

If so, you may need to provide a more appropriate file, no file, or filter some code in the page's html.

HTH