Well, those two don't always coorolate... there are plenty of wild mouse coasters where the park sets the trim on the block brake to bring the car to a crawl and there are also plenty of coasters with huge trains where the block doesn't slow the train at all. Usually, a block only slows the train if the park is using the block as a trim (frequently to slow the ride and make it so it's not so rough) or if the train ahead hasn't cleared the next block (which would result in the block causing the next train to stop altogether).
You may be confusing blocks with trims. A block brakes, by nature, cannot be uphill. That would be a trim designed to slow the train, but not stop the train. Every block ends with a spot where a train can stop fully, but still be able to advance to the next block when restarted (even if from a full stop). This can be the top of a lift hill, a brake run that is followed by a drop to regain momentum, or a launch zone.