Apart from the man page I have no further suggestions.

This is committed.

Updated patch fixes the formatting of the output of usage().

Fix formatting in usage().

The sanitization in the service script is mainly to decouple the users shell env from the services, so that the services start like at boot (as a real security feature, the sanitization would have to happen in the rc infrastructure so that calling the rc.d script with the full path automatically gets the sanitization). See also https://cgit.freebsd.org/src/commit/usr.sbin/service/service.sh?id=ee55fdb8fa1ce60d39bd7f2afde7d9772a2e3d58 for the corresponding log entry.

Do you agree that this is committed and the differential can be closed?

What's the status? Is this superfluous, or overcome by events, or waiting/blocked on something?

Should the man-page tcp_rack(4) be changed at the same time to mention TCP_BLACKBOX as an optional dependency (and what feature it unlocks in rack)?

As a non native speaker I would have expected something like "As an example" for the second sentence.