Is "I'll be John Brown" a common phrase?

Harold Wentworth, American Dialect Dictionary (1944) has this entry:

John-Brown, v. t. To 'darn.' [Example:] 1942

Fla. Suwannee R[iver] I'll be John-Browned"

M. F. Rosborough 'Don't You Cry.'

Robert Hendrickson, [The Facts on File Dictionary of American Regionalisms] (2000) dispenses with the euphemism:

John Brown An old term in the south for "to damn"; after the abolitionist John Brown. "Well, I'll be John Browned!"

And J.E. Lighter, Random House Historical Dictionary of American Slang (1997) refers to both the "damned" an "hanged" meanings:

John-Brown v. [ref. to the hanging of John Brown, U.S. abolitionist (1800–59)] Esp. So[uthern] to execute by hanging (obs.); in phr. be John-Browned to be "hanged" or damned. [First cited example:] 1869 Overland Mo[unthly] (Aug.) You need apprehend nothing dreadful, for boobies seldom "John Brown" each other.

The earliest instance of the term that a Google Books search finds is also from The Overland Monthly (May 1870), in an article titled "A Piny-Woods Character":

"Wouldn't White Men make head better, if they would not refuse to work in the same field with Negroes?"

"But no White Man's gwine to do it. It's born into us, stranger. No White Man as respects hisself is ever gwine to do it. He'll be John-Browned fust. But come, set up, stranger, and take a snook."

The next-earliest is from "Broad and Otherwise," in the U.S. edition of Puck (June 27, 1906, published in New York), we find this early instance of the phrase:

F'rinstance, there is my niece, who went to school over at Louderville, and has just lately come home with an intimate knowledge of 'most all the non-essentials and a tolerant contempt for the plain but serviceables—who tumbled down the garret stairs, the other day, and announced that she had abraded her patella and started a cicatrix on her sasamoid, leavin' me, as gentleman of the sun-burnt school, unable to ask her where she was hurt or how badly, and—er-ah!—well, education may have broadened her, but I'll be John-Browned to gosh if it ain't rapidly thinnin' me, tryin' to keep up with her intellectual curves and convolutions!

A couple of recent historical studies of the U.S. Civil War era have used John Browned in a completely different way—with the sense "to be attacked suddenly by participants in a slave revolt."

From James Huston, Calculating the Value of the Union: Slavery, Property Rights, and the Economic Origins of the Civil War (2003):

This attack on the institution of slavery [the Harpers Ferry raid of 1859] occurred in a state [Virginia] where the institution was born and fully established; it evoked the primal fear of slave revolts and all the horrors that attended uprisings of slaves—of the Haiti Revolution. After October 1859 various parts of the South underwent traumatic fears of slave revolts and being "John Browned." The raid, whatever its relation to property rights, powerfully evoked apprehensions for white supremacy throughout the South and made the Republicans appear even more demonic.

And from Albert Merrin, A Volcano Beneath the Snow: John Brown's War Against Slavery (2014) suggests another sense of the verb phrase "John Browned" during the Civil War era:

Slaves were also the chink in the Confederacy's armor. By 1861, many had learned about John Brown, and how he had died for them. They also knew about Abraham Lincoln, and that their masters feared he would "John Brown" them. ...

The Confederate government had counted on having six hundred thousand men under arms by July 1861. However, fear of being "John Browned" gripped the Southland. So, when the war began, governors kept state-owned weapons from the army. Fearing "disturbances among the blacks," they held these in reserve or gave them to local militia units. As a result, generals sent two hundred thousand volunteers home because they had no weapons for them.

I haven't been able to find any older instances where "John Browned" is used in this way, however, and in any case the meaning of the colloquial expression "I'll be John Browned" does not have the sense "I'll be attacked in a slave revolt."

Per Hatchet Jobs and Hardball: The Oxford Dictionary of American Political Slang edited by Grant Barrett (Oxford University Press, 2004), it refers to a hanged abolitionist and is an alternative for 'damned'.

This is almost definitely a regional phrase.

I live in Lawrence, Kansas, where John Brown was active and is a local cult hero. His face appears in pictures in bars, paintings in most prominent buildings, and plastered on t-shirts and bumper stickers. I have never heard this phrase in my life.

"Well I'll be John Brown" is quite simply a traditional Southern phrase for someone who has been 'pleasantly surprised', someone in 'amazement', or someone who is in mild shock about being incorrect.

If you know the true history of John Brown, you would understand that he was convinced that he was called by God to forcibly kill anyone that was pro-slavery. Yet in the very short time he had started his mission, his men were all killed and it all came to a swift ending. He was convinced he was right! Simply means surprised or amazed, that is why the phrase is used.

It has nothing to do with hatred of John Brown, nor racism, or hangings.

Having grown up in North Carolina, I was familiar with the phrase, "John Brown It". My mother never uttered a curse word to my knowledge and scolded my father when he used the Lord's name in vain. If she was extremely angry, she would let you know it by uttering, "John Brown It." Believe you me, this got our attention.