Layer Groups have their own blend mode called Pass Through which allows blending and transparency options to "pass through" the group and effect any layers below the group. To isolate the blending of layers within a group, change the Group's blend mode to Normal.
Layer Groups are, by default, set to display blending effects (such as opacity, blend modes etc.) just like any other layer in Photoshop. For example, if a layer in a Group has it’s blend mode is set to “Multiply”, it will be multiplied (blended) with all other layers below it. In this default state, clicking on the Group in the Layers panel displays “Pass Through” as the Group’s blend mode (i.e: any blending applied to layers within the group is “passing through” the group to be applied to the layers below it).
To change this default behavior to limit the blending between layers to only those layers within the Groups, target the Group in the Layers panel and set the Groups blend mode to “Normal” . For even more advanced blending of channels within Groups, choose Layer >Group Properties (or Control (Mac)/ right mouse (Win) -click the Group icon in the Layers panel and select Group Properties) to specify which channels to use for special effect blending of layers.