A backslash: \
Slashfic is named after the slash: Kirk/Spock
Fractions are written sometimes with slashes: 1/2
When you list alternatives in a sentence, you may say "slash": his/her
There are slashes in URLs: http://cos.livejournal.com/profile
Backslashes appear nowhere in the natural world, aside from a crufty old operating system from Microsoft and some of its descendants. Unfortunately, it seems to have gotten half the computing world into saying "backslash" wherever either a slash or a backslash appears. This creates confusion, and wastes syllables. I know syllables aren't such a limited resource and we can always make more, but conservation of syllables seems to be a driving force in the evolution of English, so we should be able to defeat this annoying anomaly.
If in doubt, just say "slash". You'll rarely be wrong (as opposed to being wrong almost all the time if you're in doubt and say "blackslash").
Please pass it on! Thank you :)