Blue Triangle, top left, next to BFB.

OK, I just had the blue triangle top left again. This time I noticed that the little triangles next to the icon for the app that requires my attention are slightly bluer than usual.

Hugh: correct; the blue colouration of the pips/triangles next to the icons in the Launcher indicate which window(s) have requested attention.

Learning the association (same as for the colours used in the interface) is quite likely going to be a subconscious activity were repeated exposure and predictable behaviour lead to the association without needing to be explicitly taught (as you've found). In a way, the blue notifications/pips can be seen as an optimisation stage: little is lost if a user does not immediately notice the hints but they aid the user without resorting the alternative behaviour of flashing something up on top of their desktop.

Of course, details such as the shape and style can continue to be subtly refined based on feedback and testing.

Hugh: When the blue triangle comes up, doesn't an app icon in the launcher pop out and wiggle as well? Both the BFB getting blue and the app icon popping out happen at the same time, and should be enough for the user to understand that an app is demanding attention.

Marking as invalid as it seems like just a misunderstanding. If there are any problems with the functionality itself (Blue triangle doesn't correspond to an app asking for attention for example) then please raise it as a separate bug.