Royal blue blinking lights in formation across North Dallas sky.At the park in the dark with my son we see a lot of aircraft. Our neighborhood is in the flight path of passenger jets flying into Love Field in Dallas, TX. We have a hospital a 1/4 mile from our neighborhood to the NW and helicopters fly in and land frequently. But all of a sudden we see 4 royal blue blinking lights in formation. Three were in an isosolese triangle shape and a fourth was off to the side. They appeared to be higher than one of the incoming jets initially, about twice as high. They were blinking in a perfect pattern one after another - all four lights about .5 seconds apart. They flew slowly North over our neighborhood at the same height it appeared until it was so far north that they almost disappeared entirely. After about 15 minutes they came closer and brighter again but were much higher up in they sky and were directly above us to the West. The forth light was trailing further behind and took off towards the West without the three lights in triangle formation. The remaining three continued to climb straight up until they became smaller and smaller until they disappeared altogether almost directly above where we were standing. The night was completely still - no breeze or wind at all. The lights made absolutely no sound.