This is another picture of noctilucent clouds taken on June 18, 2019.
I was not sure if they were noctilucent clouds because I saw lower level alto-stratus clouds scattered around the sky. I've seen noctilucent clouds before but they appeared thinner and wispier than the ones I took photos of on June 18. I thought the blue clouds could have been high cirrus clouds being back lit by the Sun which would have been 15° below the horizon at the time of the photos.
Another important observation is that these noctilucent clouds were the densest layer of this type of cloud I have ever seen. Their dense appearance is why I took a photo of them in the first place. Cirrus clouds do not appear this blue so I thought they could be noctilucent.