When you add transparency or import images that have transparency, grayscale images appear lighter than when transparency isn't present.
When you add transparency to a spread, the Transparency blend space modifies all colors in the spread's content. For most documents, this blend space is a CMYK profile. The result is that the image is now a CMYK image. This conversion is necessary to ensure that all of the spread's contents are handled consistently with regards to color conversion through all output methods. If part of an image went through the transparency blend space, and other portions went through another color conversion route, distinct color differences could appear. This effect is called "color stitching." For example:
By pushing all of the spread through the blending space, InDesign ensures that such color or shade differences are not introduced.