Beat of my blosh!

• brush: (L/R split) the paintbrush with which Issy is writing her letter, ands with which she is beating time for the music in this footnote; Issy's hairbrush

• blush

• beat about the bush: to express oneself ambiguously or unclearly

• bush: (slang) a woman's pubic hair → contrasted with smooth of slate? → see laylock blossoms in the next line

• beat: to mark time in music → there are many musical allusions in this paragraph