[Hearing the change in tone, Dahlia does indeed take several steps back. There aren't many substances that she can think of that would leave black marks like that, but hearing Szayel speak is what gets the last puzzle piece to click into place.]

You don't think.... impurity?

[Likewise, she's dropped her more flippant tone.]

If so, do we assume the source of it is Mr. Abarai? ...Or seeing as the only trace of it appears to be on the outside of the drawer, it would seem more logical to assume it was put there.... probably through the use of a contaminated object.

[She takes out her magical phone here, because yes, she has resigned herself to this role.]