The blacklist is meant to protect you from plugins that could cause problems. It's not that different from SONAR's excluded-plugins list. 64-bit plugins can be removed from the blacklist by the user. From the Cubase help file:
You can reactivate a blacklisted 64-bit plug-in by selecting it and clicking Reactivate. This causes Cubase to rescan the plug-in and remove it from the blacklist. To move the plug-in back to the blacklist, you must rescan all plug-ins and restart Cubase.
Looks like you may be
SOL for 32-bit plugins, though. A lot of Cubase users were upset about this, at it was an unannounced change. Not the first time Steinberg has angered it users with abrupt changes. Not sure what's really going on, though, as I saw this statement from a Steinberg guy: "For those who still rely on a few 32-bit only plug-ins, jBridge still works fine."
Steinberg does not explain the criteria for placing a plugin on the blacklist. That's probably because the criteria are fluid, as the engineers identify different reasons for potential plugin instability. At least, they do not seem to be favoring one vendor over another (although they did make the statement that "Cubase9 is more stable without third party plugins").