Is bleachbit "safe" - yes. Running bleach bit itself will not damage your system, however, you can do damage if you use it to remove things you should not or if you misuse it.

There is a difference between deleting a file or formatting a partition and over writing the file with new data.

In the first case, deletion or formatting, you can potentially recover the file as the data or contents have not been removed from the disk.

for details, see testdisk, photorec, and similar data recovery tools.

See also - http://www.ufsexplorer.com/und_fmt.php and https://wiki.archlinux.org/index.php/file_recovery and / other technical documents of the various tools.

This first type of data recovery may or may not use the ext4 journal.

The second scenario is where the data is over written, as is the case with your use of bleachbit. You can use several other tools to do this if you so desire.

In this case, the data is gone, beyond recovery. The guttman theory has been debunked =)

see http://www.nber.org/sys-admin/overwritten-data-guttman.html or similar