My sense is that Blake is talking about ownership, and the idea that the natural rights of people to the land and its resources is restricted by the artificial laws of man.
Charter (noun) formal written instrument bestowing privileges and rights, serving as legal evidence of them," c. 1200, from Old French chartre (12c.) "charter, letter, document, covenant," from Latin chartula/cartula, literally "little paper," diminutive of charta/carta "paper, document"
SOURCE: Online Etymological Dictionary
This was a huge issue in the industrialization of England, where peasants were ejected from the land as herding replaced farming in many areas, and were forced into the cities to find work in restrictive, unnatural, and generally horrific, conditions. "Blackened walls" is surely a reference to the soot of the London factories, as London itself was literally "blackened" in the industrial era.
I also suspect that there is a wordplay on "chart", in the sense that everything in the modern world is mapped, and that this rigid formalization restricts the sense of newness and wonder associated with discovery. Maps are, likewise, artificial representations, a means of definition and restriction.
Chart (verb) 1837, "to enter onto a map or chart," from chart (noun) 1570s, "map for the use of navigators," from Middle French charte "card, map," from Late Latin charta "paper, card, map". Charte is the original form of the French word in all senses, but after 14c. (perhaps by influence of Italian cognate carta), carte began to supplant it. English used both carte and card 15c.-17c. for "chart, map," and in 17c. chart could mean "playing card," but the words have gone their separate ways and chart has predominated since in the "map" sense.
SOURCE: Online Etymological Dictionary
It all falls under the theme of the "mind forged manacles", which is the key line in the poem, thematically and rhythmically.
SEE ALSO: The Garden of Love
Great question, btw. The use of "chartered" in this poem is something I think about not infrequently. :)