Saved my nappy ass. I though the video card was bad so I swapped screens but had the “black screen”. What I didn’t swap (until reading the answer here) was the flat cable. Once I did that, all was well. Thank you!
Apparently the client says the Display Contrast slider in the Accessibility System Preference>Display pane was not set all the way to the left. She must have been playing and fouled it up. So, unless she tells me otherwise, I'll call this question answered.