I'm sorry to hear that you are having trouble with Blackhole in Ardour. It does sound to be something specific to Ardour, given the plugin loads fine in other hosts.
Unfortunately, we are unable to reproduce this issue here, on multiple systems. I will note that we have tested on macOS 10.12 and 10.13.2 (not 10.13.4), but we are testing with the same version of Ardour and Blackhole.
Have you tried resetting the VST and AU Caches and Blacklists from Ardour? You might also try uninstalling Blackhole, launching/closing Ardour, reinstalling Blackhole, and then opening Ardour again. I have also noticed somewhat flaky behavior loading AU plugins on recent versions of High Sierra, that have been fixed by restarting the computer.
If the problem persists, please email firstname.lastname@example.org referencing this issue. If you've been in touch with the Ardour team, please try and put them in touch with us. It will likely be easiest to fix this issue by working with them directly.