When I first read this, I thought that Sayers had made up the song title, since it plays so directly into the story. The aimless, hungry loves of the young people is being contrasted with the conflicted, wordy coming-to-love of the mature couple (Harriet is in her 30s, Peter in his 40s), who find themselves back in the academic setting.
I was surprised to learn, then, that "Love in Bloom" is a real song. It appeared in 1934. Bing Crosby sang it in a movie. It was a nominee for the first "Best Song" Oscar. Sayers' novel is set in 1935. She is using what was really a popular song of the time (just as Peter's mysterious diplomatic business was recognized by contemporary readers as the Abyssinia Crisis, even if not directly referred to by the author).
Okay, that's interesting enough. But there's more I learned just recently. It turns out that "Love in Bloom" is the song that Jack Benny always played on the violin (badly) as part of his schtick. He was invited to sit in with a band one time (for real), and he played "Love in Bloom" on a borrowed violin. His performance was noted in the newspaper. The next time he walked into that place, the band struck up "Love in Bloom." It became his theme song, which he made fun of for the rest of his career by playing it off-key.
Somehow, putting Dorothy Sayers and Jack Benny in the same corner of my brain pleases me. I have made a connection of sorts. Who knew?