“Black as midnight under a skillet” is very dark, indeed. The phrase is probably cowboy talk; skillets were put to good use, and Texas midnights can be very dark. The phrase appears on several “Texas talk” websites.
 
 
Texas Talkin’ Page
Blacker than midnight under a skillet.
 
Texas Neopets
In case you’ve never been to Texas, here is a guide to a few of the more colorful expressions you might encounter there:
1. It’s blacker than midnight under a skillet = It is very dark
 
Google Groups: misc.writing
From:  Gene Royer
Date:  Fri, Oct 31 1997 12:00 am

“Why is it so damn dark in here?” I asked.
b
“It is?”  He seemed genuinely shocked

“Yeah.  Dark as midnight under a skillet.”
 
1 March 1968, Northwest Arkansas Times (Fayetteville, AK), “Hillside Adventures” by Fred Starr, pg. 4, col. 2:
The night was black as midnight under a skillet, and we were not yet old enough to be brave in the dark.
 
30 July 1998, Syracuse (NY) Post-Standard, Northwest section, pg. 17:
Cowpokes young and old are sure to enjoy Susan Lowell’s retelling of a well-known childhood classic, retitled “The Bootmaker and the Elves.”
(...)
The bootmaker and his wife are astonished to discover a finished pair of beautiful boots—“tall, shiny and black as midnight under a skillet.”