Blenheim 'Blind Flying Apparatus"

All

I was wondering if anyone familiar with Bristol Blenheim (or other aircraft) would explain how ‘Blind Flying Apparatus” could become un-operational at height (around 10,000 feet) due to ice formation. This would be in heavy storm and in clouds

” For approximately quarter of an hour he flew at 500 feet and then started to climb above the rain clouds.
These however, were very much thicker than he expected. At 10,000 feet he was still in the clouds and it was then that he
lost the use of his blind-flying apparatus through ice formations. He had expected to get out of the clouds at 4,000 feet.
“I continued to climb thinking I must be getting almost to the top” he went on
“but I did not see anything from the time we went into the clouds at 600 feet until we came out at Mildenhall. In endeavouring to come out of the climb I must have dropped a wing. The ‘plane started to dive.” He felt certain that from both the noise and
feel of the machine and after trying in vain for three or four seconds to recover he decided that they had better make preparations to leave the plane.”

This occurred to a Blenheim in June 1939 (L1253) piloted by Squadron- Leader lan MacKay – The W/Op was killed when he hit the tail on bailing out . Squadron-Leader lan MacKay managed to get the aircraft down the Nav/Obs bailed out OK (top hatch)

Thanks

Paul