Dragon eggs do respawn after a while. So all you need is one near Moder and move it to a box by the summon area and wait for another to spawn etc.

Now the blue swirls in the BF I see as being unrelated to the actual thistle. IDK Most of the thistle I collect has nothing to do with any swirls.

What has me wondering is that if anything player built is near it might stop spawning or if just the player being too close too often can reset the respawn timer. So if you are constantly checking you may stop it from spawning at all.