Some examples with text over a transparent background:
See the transparent black background
See the text on the shade of cyan
What would you call this type of effect?
I am not sure if they have name. The effect is different in the example sites you have given. In the first one, it almost appears as a lightbox effect with text instead of an image. I am referring to the box that has this text: "Pietro Mellini..." You could call that a "transparent overlay box".
But wait a while and someone will provide with more accurate answer.