Blake's first mention of mills says "Get to thy Labours at the mills" which doesn't sound very church like https://upload.wikimedia.org/wikipedia/commons/8/8b/Milton_a...

I looked him up and funnily enough he lived in the same area I do. The majority the churches are not gothic eg. St Pauls.

Also he lived at a time when cotton mills were booming which do look kind of dark and satanic https://en.wikipedia.org/wiki/Cotton_mill#Remodelling_and_ex...