They don't care if you can or can not actaully control the plane by reference the/a natural horizon only that you don't interfere with IFR traffic; actual sky conditions don't matter [legally] the FAA can't be there to know that
if you go up on a hazy day find yourself in IMC on the guages due to haze or night but you can see the other aircraft [legal reported vis] and you meet the cloud clearance requirements then feel free to spiral into the ground as you wish
same goes for rain on the wind shield