Get rid of the old .dll. Or at the very least, temporarily move the .dll to a location where Cubase won’t do a scan such as your desktop. When you installed FM8 you should have only installed the 64 bit version assuming you use Cubase. Perhaps you installed the 32 bit which Cubase then tosses to the blacklist folder?
Make sure, via the Cubase Plug-in manager, that it scans the location of the new version. Then do a scan. I’m not sure why you keep old versions or old .dll’s of the same plug-in, but sometimes that is necessary such as when using Kore.
After you get this sorted out, you might want to do an analysis of other duplicate .dlls where one version is older than the other. You can do that with a Windows 7 search.
Or, you can do this with The Ultimate Plug-In Tool.