I mean, there had to be some hard feelings. Black Canary first appeared as a masked crook in one of Johnny's stories in FLASH COMICS# 86 August 1947. She returned a few times and explained she was a crimefighter who posed as a criminal (like the Green Hornet). Within five issues, she got her own strip that replaced Johnny's. (He had appeared every issue since FLASH COMICS# 1 in 1940.)
Was Johnny bitter? Maybe not, he was obviously so smitten with the Canary that little hearts circled his head when she was around. Johnny was not the sharpest super-hero of the Golden Age, he was --what's the word? stupid. He was a comedy relief character, a Jerry Lewis-type clown who had control of a powerful genie-like being called the Thunderbolt. At firat, Johnny didn't realize he could summon the Thunderbolt by pronouncing the mystic Bahdnisian words "Cei-U" (which remarkably were identical in sound to the English phrase "Say you.") After he figured this out, Johnny's whacky outlook and short attention span made him barely survive his adventures, even with a magical being protecting him.
As if usurping his monthly strip wasn't enough, the Black Canary muscled in on Johnny's membership in the Justice Society. Again, Johnny had been a founding member of the group in ALL-STAR COMICS# 3, in 1940. Black Canary turned up to offer her help to the group at Johnny's invitation. Look how suspicious the members are. Maybe they still thought she was a crook as first reports of her activities said. Now, the JSA at this time had (for whatever reason) a limited roster of seven active members and Black Canary appeared as only a guest. That is, until ALL-STAR COMICS# 40, May 1948, where Johnny Thunder was gone without explanation and the Canary was a full member. One can only wonder what sort of scene went on. Did the JSA inform Johnny Thunder that his services would no longer be needed? Did he offer to step down at Black Canary's urging? Did he choke back a tear as he shook hands for the last time with the other members, heading out the door unnoticed as the Justice Society welcomed the Black Canary with her swearing-in? We may never know.