Blue Light

The reason most newer phones and monitors have the option of a blue light filters, is because blue light can penetrate all the way to our retina (the inner lining of the back of our eyes). This is because our eyes are really bad at blocking blue light. Thus excessive exposure to blue light can damage the light sensitive cells in the retina. In other words, possible degeneration of vision.

However blue light is not completely bad as it can boost alertness, memory and pretty much all cognitive (anything mental) functions.

Just keep in mind that anything can be bad in extreme amounts. Even seemingly harmless things like drinking water. As long as you don't binge read/watch anything too often, you shouldn't be too worried.