After reading Rosanna's letter, Blake tries to account for how the paint got on his nightgown without him know

After reading Rosanna's letter in the moonstone blake tries to account for how the paint got on his nightgown without his knowledge. Why does Betteredge say that neither drunkness nor sleepwalking can explain it? He asks mr.betteredge if he had been " drunk on the night rachel's birthday?" WHat does Mr. Betteredge say

...i dunno...but you can give me best answer since i did actually answer...?