To block visible light (not IR) from the camera, you need a special IR-block filter attached to the camera.

There are different materials that you can use for this purpose:
• IR-pass filter
• IR-band pass filter
• Remote control filter
• Black photo negatives

IR filter suppliers:
Acrylic filters:
http://www.knightoptical.co.uk/acatalog/FiltersColourfilters-Acrylictype50x50mm.htm

Glass filters (more expansive)
http://www.edmundoptics.com/onlinecatalog/browse.cfm?categoryid=10&level=2

IR-pass filter
An IR-pass filter is a special filter that only passes through IR on a certain frequency (760 nm to for example 1000 nm). So it basically blocks out all other light, that’s not within that region, that would be most of the visible light.

IR-band pass filter
The IR-band pass filter only passes through IR on a specific frequency.
For example: 880 (nm)
So if you’ll use 880 (nm) IR-LEDs and you use an IR-band pass filter of 880 (nm) as well, you’ll get the best optical performance, because all the light is blocked out, except for the specific IR from the LEDs.

I haven’t tested the IR-pass filter nor the IR-band pass filter, because they are relatively expansive. 80 euro for a IR-band pass filter. But from what I’ve heard, they work very well.

The IR-band pass filter works better then the IR-pass filter.

Remote control IR filter
The filter on the front of remote controls (deep dark red/black piece) isn’t very handy to work with, because of the different shapes it may have and the result is almost the same as one layer of dark photo negative.

Black photo negatives
If you have a low budget, I reckon you use several layers of entirely black photo negatives. You’ll probably know them. They are usually among the negatives of your holiday pictures etcetera. It’s those completely dark negatives that appear when pictures are misfabricated. Three layers will do the job nicely. If you use too many layers, your blobs become less visible as well.