Blocs uses srcset for its retina graphics, I’m not 100% sure when the x3 is used by the browser, it was my understanding it was used for 4k+ displays. It’s standard html5 markup so maybe some browsers aren’t taking advantage on it yet.
As for the blue box, that sounds like a bug. If you clear the X3 image does the image then show? Blocs uses the safari WebKit engine so maybe it’s picking up the 3x image but failing to load it for some reason?
I don’t have a 5k iMac to test on so feel free to fiddle around there and see what makes the image show up on the design canvas.