As a follow-up, the roofer informed me the bathroom exhaust fan was not properly attached to the tubing. Hence the accumulation of moisture/mold between these 2 rafters in particular! (and nowhere else except for adjacent to the skylight). One last photo of black discoloration on underside ...
If you don’t have an exhaust fan, put a dehumidifier in your bathroom to absorb moisture, or a standing fan to circulate more air. If you have a shower with a glass door or walls, use a squeegee to remove excess water each time you bathe. Dry any areas in your bathroom where mold...