I know grizzlies will happily step off to the side of the trail, let you pass without seeing them and then attack from behind, but I haven't heard of that many blacks doing it.  It's not a bad idea to turn around and look behind you every ten steps or so if you're in bear-brush.
Back in the 50s and 60s a lot of surveyors used to be attacked when working out in the woods.
When 'bear spray' first came out years ago we looked at it and howled with laughter. 
"Haha, that's a good one!"
"Yeah, like mosquito spray! Hahaha"
We honestly and truthfully thought it was a joke, like 'fart spray' or 'stupid spray'.  Then we got a little worried, "What if some fool actually takes this stuff seriously?"
"Yeah, heck, the only good 'bear spray' is 3006 caliber!"
Of course you have to remember this was a bunch of old sourdoughs back in the old days talking who had lived in the pristine Alaskan wilderness for decades.

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