What is a black widow? Well, besides the species of spider, a black widow is a woman whose husband has died, and she's still in constant mourning. Also, she probably killed him. So you see, black widows are creepy and perfect for a Halloween costume.
The first thing you're going to need is funeral garb. You can probably easily find something dreary and black at a thrift store. If not, visit your local mall and any of the department stores will probably accommodate you.
To make it more dramatic, you could rip up the clothes and make them look like rags (assuming you didn't pay too much for them). You can also sprinkle them with fake dust to make them look old. The nice thing about this costume is that after Halloween is over, you can reuse it for other sad occasions where you might need to wear black.
• The Veil
Just as the black widow probably wore a veil when she got married, she'll be wearing a black veil when she's mourning her murdered husband. If you're having trouble finding a black veil, look for some black tool (the kind they use to make crinoline) at the fabric store and make your veil out of that.
Make your face look pale and underfed. If you're into obvious metaphors, you could also draw a little spider on your cheek for good measure. If you want to freak people out, then glue on a plastic spider and put cob webs in your hair.
Better yet, carry a handful of fake spiders and throw them at people.
You could also carry around the knife / gun / nuclear warhead that you used to kill your husband. Cover it in fake blood. Want to give your husband the kiss of death this year? Hint at your potential murderous betrayal by dressing up as a black widow!