Given how the rest of the Interface works, I'd say that blue is used as the mask colour here, meaning that the engine will replace it with a darkened, semi-transparent version of the main HUD colour.
As for the bits where you didn't know what they were, I am somewhat stumped as well. I remember seeing them, but I don't know where. I also know that they are not mentioned in the original Manual for FS2.
Oh, and the infinity symbol is probably something in the original font FS2 uses.
EDIT: Ah, now I know what they are. If your mod uses the FS1-style HUD, those will replace the threat indicator display. The triangles show you which secondary bank you have selected, the circles do the same for primaries.