Definition: A company moved to a cloud provider in a rush without fully understanding its advantages and without fully understanding the company's own disadvantages. The results are a catastrophic disruption to the company and a heavy financial cost.
Example: "We tried to move to AWS and now a dark cloud is hanging over us."
"Trying to make your own private cloud with X technology is just a dark cloud."
A company deploying any kind of setup, cloud or not, while handling sensitive client data without having done any sort of risk assessment and due diligence is simply negligent and is a sitting duck. This does not require a fancy new term.
Possibly these folks think it's clever (tis a touch) to refer to problems with the cloud as being a dark cloud... like a dark cloud on the horizon.