Upside down cakes are a good technique for fruit that needs to cook down significantly in a cake. Apples are foremost, apricots too, plums and perhaps pineapples although the last mainly for the benefits of caramelization.
You wouldn't think very soft fruit or berries will suffer this treatment well, the end product effectively becoming a cake with a layer of jam on top, when inverted. So I was a bit suspicious of the idea of putting fragile blueberries at the bottom of the tin and then smothering them with the cake batter.
The effect is quite interesting though. The blueberries do turn into a jammy layer of sorts, not really aesthetically pleasing looking at the cake as a whole. But when you cut and taste it - yes, that works.
Polenta cake batter is so nice because of its crunch. And let's be honest: since anything with blueberries is gorgeous, so is this and I shouldn't have doubted it for a second. The recipe by Nigella via NY Times Cooking (soon subscription only!).