That's the thing about a pseudonymous digital currency -- there's little information that can be gleamed from the blockchain data.
There are some metrics reported in aggregate by the payment processors and others. For instance, last month BitPay shared during a discussion that they had surpassed 10,000 transactions since their launch over a year earlier. They didn't provide the number of bitcoins as well or give the average amount per-transaction. They are probably the largest payment processor but even that amount is pretty insignificant.
This data will probably be most accurately obtained through user surveys and from there extrapolate.