I think I remember seeing this asked and answered in the discord server, but since I don't know for sure if you're the same person, and since someone else might want to know the answer to this as well, I figured I'd still put this here :3
It's quite brief the way it's shown in the game and pretty easy to miss x3
Basically, after getting the Bliss ending, when you get sent back to the title screen and the alternate version of the main theme is playing, if you click on any of the menu buttons right away to start a new game, load a save file, access the extras section, or quit, you will probably miss it >.< If you go into settings or gallery, it should still be visible when you back out.
But yeah, sadly, if you press on anything too quickly, it's easy to miss and will disappear until the next time the Bliss ending is run through.
I'll put what it is below under spoilers :3
SPOILERS
So when you complete the Bliss end, once you're back in the title screen, the alternate version of the main theme will play, Blythe (orange) will be winking, and there will be something else that spawns on the screen instead of the usual orange bubbles!
Like all the little title screen changes I made that appear post-ending, it's not a permanent change, and as soon as you start a new game, load a previous save, enter the extras section, or quit the game, the title screen will revert back to the normal/original version until the Bliss end is completed again. It takes the same time for the varient spawn to show up as it does the bubbles, but they are different sizes, so you might have to wait up to 20 seconds to get a good enough view to see what's going on :3