No, not the mistress, which as you said is obviously the music industry but "the blonde that was still in his bed" was his confession that he slept with another woman. Jon himself has described the song as his confessional.
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These days the stars seem out of reach
But these days there ain't a ladder on these streets
These days are fast, love don't last in this graceless age
Even innocence has caught the midnight train
And there ain't nobody left but us these days
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