Fix a MSG_BLACKHOLE sanity check, add some comments

Authored by osa1 on Sep 24 2018, 8:21 AM.

Description

Fix a MSG_BLACKHOLE sanity check, add some comments

Reviewers: simonmar, bgamari, erikd

Reviewed By: simonmar

Subscribers: rwbarton, carter

GHC Trac Issues: Trac #15508

Differential Revision: https://phabricator.haskell.org/D5178

(cherry picked from commit d90946cea1357d3e99805c27dab1e811785a4088)