Here his purpose seems obvious enough.

The English constitution raised him from humble means through a Professorship at Oxford to a judgeship in the Court of Common Pleas.

He had been a member of Parliament and refused the office of Solicitor-General.

He had thus no reason to be dissatisfied with the conditions of his time; and the first book of the _Commentaries_ is nothing so much as an attempt to explain why English constitutional law is a miracle of wisdom.
Constitutional law, as such, indeed, found no place in Blackstone's book.

It creeps in under the rights of persons, where he deals with the power of king and Parliament.