iOS7 new/updated apps, NOT showing blue dot. Any one know why?
This isn't a particularly helpful answer but it seems from both internet chatter and anecdotal evidence (such as my own iPhone) that the whole update mechanism is a little inconsistent at present.

I have apps that have been updated and never opened that do have the dot, I have apps that have been updated and not opened since that do not have the blue dot.  Sometimes the number of apps shown needing an update is not in line with what the App Store says when you visit it.  Sometimes it says that all apps are up to date when you can easily find an update when you search manually on a title (for example Facebook and Twitter both showed as current even though I knew they have been updated to iOS7, hunting down the apps directly in the store and selecting Open kicked off an update instead).

I think that the best I can say is that there are issues, but they are hard to track down, are not the same for everyone, and are not even consistently the same across a specific unique device.  It's worth asking the question though, perhaps someone with more knowledge may be able to track down some formal bug report or similar that indicates that this is a wider issue that has been reported or is being looked at.  For now, it's a case of wait what is included in the next OS update.

(PS, whilst this does not answer your question, I feel it's worth putting up a little something to show it's had some attention to prevent me-too answers and duplicate questions - we can all revisit this one if further news becomes available)