That's tough for me to answer because all the priorities have blurbs but some of the side missions don't. Like.... I can tell a side mission is there, but there are multiple side missions in that location, so it flashes but no blurb. Though.....maybe that's why. Even the citadel will do it, unless there's a priority mission. In the end....it's confusing. :/
The side missions have blurbs, but only if you hover the cursor over the flasher. (Don't know if that's true for consoles.) I guess it was to distinguish between main and side quests?
Note that side quests which are required to advance main quests will also put blurbs up without hovering -- for instance, the rachni mission always gets a blurb since it's a possible prerequisite for activating Priority:Tuchanka. So the main/side distinction isn't even coherently applied.
Yeah, this is not a great implementation. I'd just put everything on the map myself, since I'm not a huge fan of distinguishing between main and side quests in the first place. YMMV, though; you might want more guidance than I do.