Purging a software package, reinstalling it, it remembers settings

It's because user specific settings are stored in your home directory and that isn't removed. Try to find either bluefish directory under ~/.config or ~/.bluefish directory under home and remove it to reset the user preference.

Update: As Zanna mentioned in the comment (which is now removed), it's .bluefish under home.

Some programs keep config preferences for installed software that the app will use for its installation and or use. This data is written to hidden files/folders in your home directory. You can view them by showing hidden files in Nautilus using Ctrl + H, or in a shell using ls -a to view the files. In your case, you want to remove the software and do a clean install. So, do what you were doing: sudo apt-get purge bluefish, find the program files in your home directory, and then manually remove them. Then you can do a clean re-install.

See these links for reference:

How to completely remove application

What is the correct way to completely remove an application?

Sometimes, applications use their own folders to store their settings , so you need to remove the files from those folders. To see which folders are used by these applications, search for files with the name of the application you want to completely remove. I use the Catfish File Search application catfish for this, but any file-search app that searches through "hidden" folders will do. After you find the configuration files used for the application, delete them. You should find that upon running the application, the settings have been reset to their defaults.