Always when planting trees, shrubs and other long-term subjects you want the same soil throughout the potential rooting area - no amended holes, pockets, strips or other small areas. If the mix you mentioned is that whole be area then that should be okay - unless the organic component is too high, as the dark appearance might be taken to indicate.

Next you should mulch with wood chips or other coarse material around the new tree.

Oh that's black mulch that is on there already. We did a mix of about 60 native soil mixed with the container soil/20 compost/20 peat moss. Maybe even a little less peat moss. The trunk flare is right at the surface and we kept the mulch away from the trunk as you can see in the pic.