Backup is essential for all of us with important digital data on our laptops, dekstops, and mobile devices. Choosing the best backup software is probably on eof the hardest decisions to make because the wrong choice may not be obvious until it is too late! Backblaze vs CrashPlan is probably right up there with Mac versus PC!
IMHO, the problem with Backblaze is that is has very easy backup, but restore is painful -- Fairly slow and it doesn't restore in situ back to where things came from. Most of the people that love Backblaze, have never used it to restore any large amount of files, so they have never seen this limitation - it may be years before they ever need to do a major restore.
That is pro or con, depending upon your skill level and what you want in a restore solution.
Another difference is how external directly attached drives are handled. Backblaze will delete all your backup files in the cloud that originated from an external drive if you don't connect it to your Mac at least once every 30 days. CrashPlan has no artificial limitations.
The more obscure difference is that Backblaze ignores macOS file tags, whereas CrashPlan supports them. So if you like to organize and search for files on your Mac using the operating system ability to "tag" files, you will lose all your hard work in in tagging everything should you have to do a restore. (The obvious point is that file tagging is itself obscure and very few people use it.)
The marketing noise that is made about Backblaze being a native app and CrashPlan being a Java app is more questionable. CrashPlan uses an embedded sandboxed copy of Java runtime so really, this is just a private code library inside the app and does not pose the security risk that running Java directly has.
The CrashPlan user interface is not as "pretty" and has an old-school look to it, but it works fine and is not buggy. Since typically you spend very little time in the app, it is a Ronco style "set it and forget it" type of use, that you may or may not want to choose based on the visual appearance.