It was made a test in warlords 2.08 that indicated that blue circles didn't took in account unknowed resourses ( the ones that you don't have the tech to see ), but they took in account known resources that were in fog ( still in the black ). Don't know if that applies to BtS ( no test was made so far AFAIK )
The test was quite simple: go to WB, prepare a uniform landscape ( grassland would be fine ), place resources just outside of the settler BFC and play. Notice if there is a shift of the blue circle.
I just did a series of experiments with world builder to test out different possibilities as per your suggestion:
case 1: take out the oil hidden in the south, and add an oil to the forest tile in the north
result: circle stays there
conclusion: the computer can't see the hidden oil resource.
case 2: remove the goodie hut
result: circle stays there
conclusion: goodie hut not that important
case 3: change the two clams in the north to one fish and one crab
result: circle stays there
conclusion: variety of resources not that important
case 4: switch the three plain tiles in the south to desert
conclusion: desert tiles are more valuable than seafood resources according to the AI!!!!
case 5: switch the two clams into grasslands
result: circle moves to the north!!!!
conclusion: non-river grassland tiles are more valuable than seafood resources.
Final conclusion: Don't trust the blue circle when it's near the coast. The computer seriously underestimates the value of coastal tiles. FYI, I was playing BtS patch 3.13