From personal experience (as an Oregon resident, not a drug dealer), it will be incredibly difficult for black market sellers to compete with the technology and economies of scale that large companies will bring to bear on the cannabis market. Taxes would have to be much, much higher (200%?) to allow for significant black market competition. Of course there will always be some fraction who buy "under the table" just as there will always be some people who prefer to brew their own beer but overall their impact is negligible as a proportion of the whole.