It depends on the degree of concussion to start. Concussions are physiological events with functional impairments, the results dependent partly on the brain's condition prior to the event in question and partly on the injury incurred. It's a diffuse injury.
Dural hematoma within the calvarium are a Space Occupying Lesion, they're going to increase pressure and have an impact on the areas of the brain initially contacted (remember the concept of a potential space between tissues); left to do their thing they can have a pronounced impact up to cerebellar tonsillar herniation (decorticate posturing, Cushing's triad, anisocoria, Cheyne-Stokes respirations). The brain isn't set up to contend with pressure or force as are our bones, and the downstream effects of the swelling can manifest in many places in the body in various ways. The layers wind up with hematoma for various reasons, but the sub dural tend to be slower growing and epi dural tend to be faster to develop. Blood out of place is always a bad thing, in this case it's become an unchecked inlet into a closed system and it generally causes problems.
A subarachnoid hemorrhage is an active bleed, a leak in the vasculature, that leaks into the CSF. They're normally tied to the arteries that circle the bottom of the brain and tend to be rapid onset. These are surgical cases. Brain tissue isn't designed or evolved to be bathed in blood else hemorrhagic strokes would be a good thing.
