Here is the full list of all 21 A Boy and His Blob achievements.
Complete all the challenges of the cave world
Complete all the challenges of the forest world
Complete the cave world
Collect 30 treasure chests
Complete the forest world
Hug the blob 5 times
Complete the Blobolonia world
Complete the citadel world
Beat the final boss
Collect 60 treasure chests
Collect 90 treasure chests
Collect 120 treasure chests
Feed Blob one of each of the jelly beans
Turn into a mech for the first time
Complete all the challenges of the Blobolonia world
Complete all the challenges of the citadel world
Rescue all of the captured blobs of level Citadel 2
Rescue all of the captured blobs of level Citadel 3
Rescue all of the captured blobs of level Citadel 5
Rescue all of the captured blobs of level Citadel 6
Rescue all of the captured blobs of level Citadel 7