I’ve seen several people struggle with how to enter Blue Cave, so here it is. It is pretty simple once you know how.
The entrance to Blue Cave is located in Mushroom Grove, in the northeast corner. One of the NPC’s up to this point might have told you to remember the code 100 1010, which is the code to enter Blue Cave. There are 7 switches in front of the cave, and you must put the code in without hitting any of the wrong switches. If you hit the wrong one by accident, make all the switches red (0) again, then put in the code. You may have to exit Mushroom Grove and come back to try again, but I think resetting the switches will do the trick.
I hope that helped. Thanks again to $@GA #merkelsommer for providing this screenshot.