It's possible he's illegitimate. Middle names had come into style by then, so it's also possible he's [*] Arnold Trueblood. I'd check all the baptisms around the right date to see if they have any later records that confirm they were different people. If there's any evidence that he's a Dissenter, that would explain the lack of a baptismal record too. I'd also see if John Trueblood had any relatives, since he may have had a brother or cousin or something living with him. I found some marriage records on FindMyPast that are probably connected to your guy, including an "Arnill Trueblood" in Addlethorpe, Lincs. in 1821 and an Arnold Trueblood in Lincs. in 1832. He's also got a hit in an 1824 Lincs. death duty register, and I'd see what that's about, maybe a next of kin or something.