The problem with pressure and ears is that, as you go down, the pressure outside exceeds the pressure in the middle ear. Since pressure equalizes essentially instantly across all the fluid-containing spaces of the body, that means you have higher pressure in the cells and capillaries of your ear than in the air spaces. This eventually leads to tissue swelling and fluid exudation into the middle ear, which is why new divers so often have some muffled hearing or crackling in their ears after their dives.
Yes, blowing air into your ears when you are not descending is raising the pressure in the middle ear above ambient, but that is tolerated very well so long as the overpressure is small, and anything you are comfortable doing is probably harmless. The eardrum is very well innervated and doesn't like being stretched, so you'll likely stop blowing air into your ear well before any damage occurs.
So the whole concept of "pre-pressurizing" is to avoid the negative pressure altogether. This not only minimizes fluid accumulation in the middle ear, but it keeps equalization easier, because when the middle ear is collapsing, it often changes the geometry so that the Eustachian tube doesn't want to open. In other words, the later you wait to equalize, the harder it is to do; by blowing air into the ear even when you don't really need it, you keep it easy.
How hard should you blow? Well, certainly not any harder than you would blow your nose into a Kleenex. If you are finding it necessary to blow extremely hard, either your Eustachian tubes aren't healthy (as when you have a cold) or you have waited too long to try. It should actually be pretty easy to pressurize your ears on land, and that will give you an idea of how hard you need to blow to do it underwater -- if you're having to try significantly harder, come up a ways and try again.