Blue Steel

“Blue steel” can means many things. Steel goes through a “bluing” prcoess to protect against rust. Revolvers often have blue steel barrels. Two movies were titled “Blue Steel”—a 1934 western starring John Wayne, and a 1990 drama starring Jamie Lee Curtis.
 
In Texas slang, “blue steel” means an erection that’s very hard and won’t go down, probably a borrowing from the “blue steel” pistol usage.
 
 
Wikipedia: Blue Steel (disambiguation page)
Blue Steel may mean:

Martensite steel aka spring steel
Bluing (steel), a process in which steel is partially-protected against rust
Blue Steel (film), 1934 western starring John Wayne
Blue Steel (1990 film), 1990 thriller starring Jamie Lee Curtis, Ron Silver, Clancy Brown, Elizabeth Peña and Louise Fletcher
Blue Steel missile, the British Cold War nuclear missile
Steel blue, the color
Blue Steel is the name of a pose in the movie Zoolander
‘Blue Steel’ Guitar strings made by Dean Markley Strings, Inc. 

Wikipedia: Bluing (steel)
Bluing is a passivation process in which steel is partially protected against rust, and is named after the blue-black appearance of the resulting protective finish. True gun bluing is an electrochemical conversion coating resulting from an oxidizing chemical reaction with iron on the surface selectively forming magnetite (Fe3O4), the black oxide of iron, which occupies the same volume as normal iron. Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and galvanic action.

In contrast, the red oxide of iron (Fe2O3), does not occupy the same volume as iron, thereby causing the typical reddish rusting away of iron. Both “cold” and “hot” oxidizing processes are called bluing, but only the “hot” process provides any significant rust and corrosion resistance, and then only when also treated with an oiled coating.   
   
Steel blue
Steel blue is a shade of blue that resembles blued steel. 
 
Google Books
To the Limit: The Untold Story of the Eagles
by Marc Eliot
New York: Da Capo Press
2005  
Pg. 337:
(Blue Steel was Richard Bowden’s group, in which he played guitar. He gave the band its name, Texan slang for an erection that refuses to go down, based on the stories that followed Bowden on the road.)
   
The Kid: Weddings
He got an erection. Not just an erection but the type that we remember fondly when we’re much older: instantaneous and blue steel hard.
 
Texan Talk
A Blue-Steel Hardon
An adamantine erection. The difference between a regular hardon and a Blue-Steel hardon is: when you press downward on a regular hardon and release it, it springs back up and slaps you in the belly two or three times. When you press down on a Blue-Steeler, your feet fly out rearward from beneath you.
 
7 March 1885, Atlanta Constitution, pg. 4:
In the works of the writers who imitate Ouida, the revolver is always a dainty toy, with an ivory handle and blue-steel barrels, set, too, if necessary, with priceless opals and star sapphires.
 
4 July 1895, Los Angeles Times, pg. 9:
Or, a 38-caliber Colt’s revolver, blue-steel barrel, black handle and $20 in gold.