This is quite a thread bump, but since the topic was never really met with a conclusion, I wanted to bump it up with my own issues and see if anyone in this thread ever got answers.
I've been seeing similar issues, and like someone said, it tends to appear after 5 days.
I have some strains that are definitely prone to it, and then others that seem to catch it while rooting.
I believe it's fusarium on my side, but not 100% still. I noticed mine have purple areas near the petioles, but they do the same dark leave issue. If left, the clone will sometimes collapse too. Makes me think there may be several diseases causing similar symptoms in this thread.
My mothers show no visual problems, only noticeable when rooting.
It becomes aggravated with high humidity, and it is a trigger, but even happens without spraying the dome and giving it lots of air.