Why would "bydd" lenite when it's at the start of the sentence in a question, but not after "Ble"?
Because there would never have been a particle before it for it to mutate.
We don't change the verb to the question form after "when?", "where?", "why?" and "how?". e.g. in the present tense we say "Ble mae e?" not "Ble ydy e?".
Sometimes nos is used for evening - I've seen it elsewhere in the course - so should "where will he be thursday evening" also be allowed?