Use darker text color in blockquote

Summary: Ref T6792, make blockquote text use darkblue, not blue.

Test Plan: Quote a bunch of text

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6792

Differential Revision: https://secure.phabricator.com/D11018