to add to gordon77's note, IR LED illuminators are much less visible than normal visible lights, but they are not completely invisible if you are looking right at them with a dark-adapted eye. Visually they will appear a dull red color. The longer the wavelength, the less visible it becomes, but also the less bright the image appears on the Pi-NoIR camera because its sensitivity also falls off, reaching zero around 1000 or 1050 nm. So 850 nm LEDs are brighter (both to the camera, and to the human eye) than 940 nm LEDs. Depending on what you are doing, this may or may not matter.
If you want to take pictures in total darkness where no one will ever see your light source even if they are really trying, then you have to use a thermal-IR imager. That is a different and much more expensive class of sensor, than the common silicon-based camera sensors we use on R-Pi, cellphones, etc.
To answer your question about what the IR-block filter does, look at the two pictures below, taken with the same camera. On the left, IR-blocking filter in place, and on the right, no IR-blocking filter (and also an IR-pass filter). Most people like their colors to look like the one on the left
and to do that you need the IR-block filter. Every consumer color camera includes one, apart from the specialty items like the Pi-NoIR model.
On the other hand, without the IR-block filter you can see how IR-bright the green plants are, something we don't normally see, so it is an interesting effect. The photos were taken with an old Sony TR7000 "Digital-8" video camera which has an IR filter than can be mechanically flipped into or out of the optical path.