"Since the World of Greyhawk included Blackmoor, should it go there (AD&D1e)? Or since I was drawing from OD&D & supplements, should Sup2 (Blackmoor) go there? Anyway, it ended up in BECM, as you know."
TSR already had plans for a series of Gazetteers, each detailing one of the Known World countries, featured in the world map in Mentzer's Expert set. But it was decided not to make a Gazetteer for Blackmoor:
"Given the importance and nature of Blackmoor -- the first fantasy campaign, to some -- it got its own line, instead of being a 'mere' gazetteer."Bruce A. Heard was product manager for the Classic D&D line at the time. As Mentzer was busy with other projects, he had to look to other authors to write the setting material. Aaron Allston, who would later make a significant impact on the Mystara setting, began writing the first Gazetteer. For Blackmoor, Heard got in touch with a man called David J. Ritchie. The result was a series of four adventure modules written for levels 10-14. The legendary fifth module in the series has been discussed earlier. The module series were labelled DA, the Dave Arneson modules.