Jesus used this question to get the people to think about something that they already know. This can be written as a statement. Alternate translation: "We all know that a blind person cannot guide another blind person." (See: [[rc://en/ta/man/translate/figs-rquestion]])
Some languages might prefer, "if one did." This is an absurd situation that is not likely to actually happen. (See: [[rc://en/ta/man/translate/figs-hypo]])