Is a white/black deck viable?
Is this viable? Yes.
Will this always be the best choice? No.

An example of an old deck which I played in the past:
Black and white knights (pro black on the WK), pestilence, circle of protection black (old rules: pump pestilence once with 2-3 mana. Power CoP black once. A castle or two, sengir vampires and serra's if you can find them. Pegasi, (with castle they survive 2 damage), ... hypnotic specters, unholy streght, terror, ...

That deck would not survice and serious competion, but against similar decks of starters it did quite well. So yes, depending on your environment you can build quite viable decks.

If you move up en anvironment where people are actually buying current generation cards to build a tournament deck, then the answer if different and it will vary between releases of new expansions.

But as I read it you are just starting with MTG and a agressive black deck with some help from white can do just fine.