Layer Groups have a unique blend mode called Pass Through which is only visible when a Layer Group is targeted in the Layers panel. It allows any adjustment layers, blend modes, advanced blending options, opacity and fill values applied to layers within a Group, to affect layers below the Group in the layers panel. To restrict the blending of layers to only happen within a Group, change the Layer Group’s blend mode to Normal.
In this example, I added a Black and White Adjustment Layer to make the two layers - the hat and coat and tie, in the Layer Group to display a black and white. However, because the Layer Group’s blend mode was set to it’s default “Pass Through” blend mode, the Black and White Adjustment layer passed through the bottom of the Layer Group and affected the Background layer as well. Changing the Layer Group’s blend mode to “Normal” restricted the Black and White Adjustment Layer to only affect those layers within the Layer Group - allowing the Background layer to appear in color.