Later gospel writers were quite uncomfortable with Mark's account of John the Baptist baptizing Jesus, and so included that pericope in spite of their purposes, which would seem to make it historical.
On the other hand, we have to be careful in applying these criteria here because just because Mark's baptism pericope made later writers uncomfortable, that doesn't mean Mark was uncomfortable with the baptism pericope: Mark says “The beginning of the gospel of Jesus Christ ; as it is written in the prophets.” Mark immediately interprets John the Baptist as a forerunner of the Messiah (a la Elijah in II Kings 1:8). Mark then clothes John similar to Elijah (Mark 1:6. II Kings 1:8.) He then says John ate locusts and wild honey,the food of the wilderness in which Elijah lived (and so on and so on). Some see in the Jordan baptism and the endowment with the spirit a repetition of 2 Kings 2, where, near the Jordan, Elijah bequeaths a double portion of his own miracle-working spirit to Elisha, who henceforth functions as his successor and superior. So there isn't really any reason to suppose the historical Jesus was baptized by John.
On the other hand, some have tried to rescue the historicity of the Jesus/John relationship by pointing out John doubts Jesus, but this wouldn't seem to be any more of a guarantee of the historicity of Jesus' relationship with John the Baptist than it would be arguing on analogous grounds that Peter must have denied Jesus just because the story says so, or Thomas must have doubted Jesus, or the disciples actually did doubt what Jesus could do in certain situations when he had just performed a miracle resolving an analogous situation just a little while earlier, etc.
Later gospel writers were quite uncomfortable with Mark's account of John the Baptist baptizing Jesus, and so included that pericope in spite of their purposes, which would seem to make it historical.
On the other hand, we have to be careful in applying these criteria here because just because Mark's baptism pericope made later writers uncomfortable, that doesn't mean Mark was uncomfortable with the baptism pericope: Mark says “The beginning of the gospel of Jesus Christ ; as it is written in the prophets.” Mark immediately interprets John the Baptist as a forerunner of the Messiah (a la Elijah in II Kings 1:8). Mark then clothes John similar to Elijah (Mark 1:6. II Kings 1:8.) He then says John ate locusts and wild honey,the food of the wilderness in which Elijah lived (and so on and so on). Some see in the Jordan baptism and the endowment with the spirit a repetition of 2 Kings 2, where, near the Jordan, Elijah bequeaths a double portion of his own miracle-working spirit to Elisha, who henceforth functions as his successor and superior. So there isn't really any reason to suppose the historical Jesus was baptized by John.
On the other hand, some have tried to rescue the historicity of the Jesus/John relationship by pointing out John doubts Jesus, but this wouldn't seem to be any more of a guarantee of the historicity of Jesus' relationship with John the Baptist than it would be arguing on analogous grounds that Peter must have denied Jesus just because the story says so, or Thomas must have doubted Jesus, or the disciples actually did doubt what Jesus could do in certain situations when he had just performed a miracle resolving an analogous situation just a little while earlier, etc.