Interesting thing about Balassa-Samuelson is that it is a fairly recent phenomenon. If you look at the data for the 50's and 60's there seems to be no relationship between relative price levels and relative per capita income. Then from the mid 60's or so the regression line's slope begins to increase (in abs value) so that by the 80's it's about as established a fact as empirical economics can get you.
All this suggests that TFP growth in richer countries has been biased towards export/tradable sectors. The question is why? (And in fact some intuition might suggest that the opposite should be the case)