Blood Potato is one of those games where equal enjoyment can be had by both participants and observers (at least a couple of whom are essential to avoid injuries). It's a very simple game, simply a psychologically more disturbing version of 'tag' crossed with 'murder in the dark'.
Actually, you don't need any equipment as such, but here's a list anyway...
A relatively large group of participants, preferably teenagers or students.
A couple of observers, preferably sober and sensible.
A large indoor space, as free as possible from obstructions, where natural light can be blocked and artificial light switched off.
... and that's it.
Block/turn out all light sources as far as possible. A twilight feel will do, but broad daylight just won't work.
Have all participants close their eyes. One of the supervisors (normally known as adults) must then select one of the participants by patting them gently on the shoulder. Wise people will do this in the midst of some random wandering around to make the location of this person less obvious to the other participants.
The observer-supervisors must now clear out of the way.
The participants, upon a given signal (eg 'Go!') wander around slowly, eyes still shut1. If they happen to bump into someone else, they must murmur the word 'potato' and carry on. Unless they are the person selected above, in which case, the word 'blood' should be murmured instead.
If a participant bumps into someone and has the word 'blood' murmured at them, they must scream, open their eyes and move out of the way of the other wanderers, probably joining the observers and other 'bloodied' participants.
The game continues in like manner until one of the following outcomes is reached:
There are only two people left playing and the 'potato' who is left is declared the winner.
The observers are feeling cruel and leave things running as a tense cat-and-mouse situation between the two of them for as long as everyone's nerves can bear it2.
Hmmm, That Doesn't Sound Too Involving...
Perhaps not, but this Researcher has found it to be a very engaging, popular game. There is something thrilling in wandering around in complete gloom hearing a chorus of potatoes and screams. And when the herd begins to be thinned, it is fascinating as an observer (or former participant) to watch how the participants try to avoid each other, relying entirely on the sense of sound and/or a mysterious sixth sense3.
Variation - the Heavenly Chorus
This variation gives those who are soon got by the potato killer (or whatever it is, no-one knows, and no-one cares) something to do.
When sufficient people are 'out' (having had 'blood' murmured in their ear), they form a 'heavenly chorus'. This means that they make a nearly-musical 'ah' sound, which increases in volume as the 'blood' person approaches a 'potato' person. Much like the 'dah-dum' in the Jaws soundtrack. This is mildly amusing for the chorus and increases the tension for those still playing. As the volume increases, the 'blood' person knows that s/he is closer to his or her prey, and the 'potato' people panic. Often two potatoes will be heading toward one another, hear the volume increase and panic - one will then almost certainly bump into the 'blood' person4.
This game should not be played by anyone suffering from nervous tension, as the whole thing seems to be designed largely to cause fear in those whose nerves are made of anything weaker than steel. The darkness combines with the slow pace of the game, the shuffle of other people's nervous footsteps and the quite unnerving background potato-ing to create a ridiculous level of terror.
Which is what makes the game so much fun.