capitilization by proxy

Try perusing some William Blake, moose, or other poets of that period, and you'll see arbitrary capitilization with no rhyme or reason (is that a redundancy?). Anyway, I've been a Blake devotée for years, and I could never figure it out or find any citations to clear up the reason for it. I know it's derived from the Middle English forms. How long did this proxy capitilization continue? Was there a grammatical precedent for capitilizing some nouns and not others? Was there, at some point, some orthographic decree that put an end to the practice in English completely and immediately?...tsuwm? Bingley?