Blend modes work differently depending on whether they’re used with groups or layers.
When a group is set to Pass Through (the default blend mode for groups), each layer in the group blends with all layers and groups that appear underneath it in the Layers list—including layers in other groups. Pass Through is available only for groups.
When a group is set to Normal, each layer in the group blends with all layers that appear underneath it in the Layers list—within the same group.
When you set a group to any other available blend mode, the following happens:
The layers in that group are blended according their own blend modes.
The resulting composite of all layers in that group is then blended with other groups lower in the Layers list, according to the selected blend mode for the original group.