The Black Stranger as written did not fit the chronology that de Camp believed in. De Camp wanted Thoth-amon to be a foil for Conan, so he wanted Thoth-amon in the story. Also, the original ending did not suit de Camp - he wanted the story to lead right into Conan's usurpation of the throne, instead of as a springboard to another pirate career.
De Camp rewrote the part of "Wolves on the Border" that establishes that nearly a six to ten years had passed since "Beyond the Black River" and Conan's assumption of Aquilonia's throne. He wanted a faster transition than Howard did.
De Camp's Chronology: Pool of the Black One, Red Nails, Jewels of Gwahlur, Beyond the Black River, Treasure of Tranicos, Wolves Beyond the Border (within weeks or months of Treasure of Tranicos), Phoenix on the Sword.
Rippke's Chronology (using only Howard sources): Pool of the Black One, Beyond the Black River, The Black Stranger, Red Nails, Teeth of Gwahlur, Wolves Beyond the Border (six years or more after The Black Stranger), The Phoenix on the Sword.
Mongoose's Conan, at least in the books I write, uses the latter chronology. Marvel's Conan comic used de Camp's chronology; Dark Horse's Conan comic, so far, has been using Rippke's.
Interestingly, Howard rewrote the story himself as a non-Conan story starring Black Vulmea the Pirate when The Black Stranger failed to sell. Howard's rewrite can be found in "Black Vulmea's Vengeance."