Found this on google as well, so it appears Old Harry is the Devil.
'There is an old Sussex saying 'as black as the Devil's nutting bag', which is associated with the superstition that it is extremely unwise to gather nuts in autumn on a Sunday because that is when Old Nick is himself out nutting. Generally people do not go nutting on any Sunday in autumn because you might meet the devil gathering nuts.'
When rainclouds are gathering, as well as saying its black over Will's mother's my mum use to say it was 'as black as Newgates Knocker' (Newgate prison). I always thought she was saying nuggets knocker as she had a Yorkshire accent.