Maybe for business/organizations that declare what they made to the state in the first place. But how does the state demand for something of which they did not know took place between individuals? By going around pricing the market value of everyone's assets and demanding payment in they way they want or get thrown in prison? Or by going around to individuals singled out by incumbents?

They do this the same way they demand taxes on payments made only in cash today. Is it foolproof? No, but it doesn't have to be, just as taxes still ended up coming in somehow even when business ran mostly on cash and cheques.

In this case the blockchain is visible for all to see, including the governments, and they only have to resolve the true identity of someone once to be right for all time.

For business it will be the same as it always has been: You are naturally under the regulation, effective or not, bitcoins or not, of government when you establish oneself as a business. They can use the blockchain to help, but let us not pretend that in itself is a panacea to taxation required for the gears to keep grinding on in the central banking/ governmental taxation systems (which even under loads pre bitcoin, were faltering).

But for individuals, one can create as many wallets as they wish, use as many laundering clients over tor, to remain out of reach. Which is exactly why governments have to set up honey pots to confiscate bitcoins on sites like the silk road (which end up not being effective deterrents and end up being outed as such by the community that relies/favors on reputable past trades). I suspect business/organizations can employ similar technologies to reduce their taxation and how much they declare.