Why I Oughta...

I have some light-green colored (new) patches of grass in our backyard right now.  When I pull up a section of it, this is what it looks like below:  There is A LOT of root-structure at play, isn't there?  Is this a weed - like the dreaded Poa Annua?  Or...could this be Kentucky Blue Grass that I germinated and planted last year?  The color...of the patch (lime-ish green) has me worried that it is Poa. Any grass experts out there that can confirm what I'm dealing with here?  And, what I should do? One thought I had was to try an application of Tenacity?  It doesn't harm KBG and turf-type Tall Fescue .  So, it if does *anything* on this grass, that tells me it is Poa? 

Last week, I shared here on my lawn diary a little breakthrough.  For the past 18-or-so months, I've been watching a grassy weed creep along and begin to migrate from my far backyard to closer to our patio.  And, after reading about various grassy weeds, I came to the (initial) conclusion that I was dealing with Poa Annua - or an annual Bluegrass.  It was showing some of the signs like being lime green.  But, after observing the lawn this Spring, I noticed that the parts affected green'd up later and seemed to handle the Summer better than the balance of my Bluegrass.   I was planning on doing a complete renovation in the back by killing EVERYTHING off and starting from scratch with a new layer of seed this Fall.  As I prepared for that, I went in to see if I could learn more about what was back there and how much I *really* needed to kill off vs. just overseed. When I went in and pulled a mature stalk of grass, I discovered it had this sort of horizon...