A folder named Bloom is added to your computer when you install the Bloom program. It contains one or more subfolders. Each of those subfolders is a collection.
In
the
Collections
tab,
right-click a book and then click Show
in File Explorer.
Leave the computer file browser open, but close Bloom.
Go to the Bloom folder:
Windows® - My Documents\Bloom or Documents\Bloom
Linux - /home/<user name>/Bloom.
Find the subfolder that has the name of the collection that you want to delete.
Delete that subfolder.