Rich block is 100% in CMYK, auto black is CMY is 0 and black is 100%
Rich black, in printing, is an ink mixture of solid black over one or more of the other CMYK colors, resulting in a darker tone than black ink alone generates in a printing process.
Rich black is used when you have a large area to cover. Auto black is used for smaller elements such as text and little areas. Rich black is "blacker than black" and uses different RGB combinations to achieve a darker look, whereas auto black is only 100% black.
Rich black includes a mix of hues in addition to 100K. Auto black may be either 100K or rich, depending on the software's default settings. It's always important to check out what that is. I prefer to always work on a screen with the settings displaying the actual black.
Rich Black has a combination of Cyan Magenta Yellow and Black and the auto black only contains 100% black. Rich black is darker that autoblack and is used to cover design that has multiple color layers to avoid black transparency.
Plain black is just 100% plain black ink (K=100); rich black is a combination of inks to create the black (C=63,M=52,Y=51,K=100)
Rich black is made up of more than one color usually a percentage of each color CMYK which produces a richer black than auto black which is only black ink.