The icon sets have been updated for use with Blocs 2.4, this makes them non backwards compatible. In order to use Blocs 2.3.2 and 2.4 on the same Mac you will need to do the following after you have run 2.4 for the first time.
• Go to this location Library/Application Support/Blocs 2/icons/, you will see 5 icon directories within it.
• Now download this and unzip it. If you look inside you will see the same 5 directories. Within each directory there is a file labeled collection.json. Copy each of these files from the respective directories into the same directory that resides within the Library/Application Support/Blocs 2/icons/ area on your Mac.
As of Blocs 2.4 the icons are loaded from another file called iconSetData, Blocs 2.3 and lower loads the icons from the collection.json file. All you have done is make sure Blocs has access to both file types. So within each icon directory you will now have a iconSetData file and a collection.json file along with the font and style directories.