Dave Arneson's Blackmoor gives us the following dates:

994: Marfeldt the Barbarian begins his legendary rampage. First Fantasy Campaign tells us he killed the wizard who created him during a "friendly wrestling match."

996: William of the Heath picks up the sword and armor beneath Castle Blackmoor and becomes the Blue Rider.

First Fantasy Campaign recounts an alternate story, however, in which a "demented old wizard" gave a passing adventurer the sword, armor, and warhorse in order to aid him against one of his own creations. It's implied that the wizard is dead, probably at the hands of the Blue Rider.

However, what if the rebellious creation was Marfeldt? Given the dates above, Marfeldt's creator could have gifted a passing adventurer with the Blue Rider's equipment in a desperate attempt to prevent Marfeldt from killing him. Whether it was Marfeldt or the Blue Rider who ultimately caused the wizard's demise, the Rider could have subsequently been killed beneath the twelfth level of Blackmoor Castle only to have William of the Heath discover the items there a few years later. That would explain the two contradictory stories; there were two Blue Riders, a short-lived one and William.

Are there any likely candidates for who this wizard might have been?

Svenny was with the Blue Rider when he got his sword, armor and horse. We were deep in Blackmoor dungeon. The encounter had nothing to do with Marfeldt. I am not sure about the story related to Marfeldt's creation and who was involved. The guy who played Marfeldt is the guy who first introduced me to Dave Arneson, but I don't recall ever playing Blackmoor/D&D with him and don't know those stories.

Sure, the blue armor etc. could have been used by someone before Bill, so no harm in tying the three stories together. 996 and 997 cover the year 1972, pretty much and Bill Heaton appears to have gotten the blue armor very early in 1972 or late 71.

Interesting! Thanks.