Heard on the radio today, "negative backlash". Isn't that redundant, or can backlash ever be positive? ~

In a physical sense, yes. It is a recoil between two or more mechanisms. It could be positive or negative depending on what needs to be achieved in the machine.

IE: backlash against bigotry could be either "negative" or "positive".
Negative against the person, or ideals of bigotry.
Positive in favor of community activism, or working toward a better future, or education against bigotry.

There is nothing about backlash that suggests negative or positive.

Backlash is an unintended consequence of some action. It can be good or bad. but is most often considered bad by the person it happens to, because that was not what he hoped to achieve.

it could probably go either way

Yes, but people only talk about it when it's negative. When it's positive they call it something else.