While you should be extremely careful when seeing one they are not aggressive and will only bite when faced with a threat. Of course the speeding boot of a gardener could be perceived as a threat. Very young children and elderly adults are have the greatest risk when being bitten by a black widow. In fact less than 1% of all black widow bites are fatal. The bite itself is usually painless and not noticeable but the toxin that they inject can be extremely dangerous. Among several other possible symptoms black widow venom can cause leg cramps, muscle cramps in the abdomen, nausea, vomiting, sweating and muscle tremors. Not a very pleasant experience!
Black widow females are all black with a red hourglass shaped marking on their abdomen. Unfortunately I couldn't get a good shot of that feature, my subject wouldn't cooperate. I was gathering rocks to use for stone edging in my garden and flipped a larger limestone rock over and found this black widow. I suspect that the safest way to sort through rock piles would be to turn them over and check for black widow spiders before picking the stone up. Even though bites are rare, I still don't want to be in that 1%!
An interesting fact that I read on the Virginia Cooperative Extension fact sheet about Black widows is that the number of bites from black widows has probably dropped since the use of outhouses declined! Apparently black widows like to nest in outhouses since they are typically warm and dry places with lots of insects. It's just something you might want to think about the next time you're in the outhouse!