I didn't see any examples of negative "boost". I feel like I'd much prefer to maintain the diversity of the internet by discouraging the dominant crappy sources than by boosting up the finite number of quality sources I can think of
It is true that there are currently less examples of negative boosts, but it is supported by Goggles thanks to the 'downrank' option.
For example:
$downrank,site=example.com
If you want to be stricter, you can also discard sites completely of course:
$discard,site=example.com
I hope that helps,