In NXRM3, the blobstore contains all the components of your repository manager. If your disk is full, you will not be able to write anything more to NXRM and risk corruption of existing data.
Cleanup can be performed using scheduled tasks. What you need varies based around what formats your system is using. You can find more general information here: https://help.sonatype.com/display/NXRM3/System+Configuration#SystemConfiguration-ConfiguringandExecutingTasks
It is important to note that you must run the "Compact blob store" task after any cleanup is done, otherwise the space will not be freed.
However, it is advisable if you have reached full disk space, you shut down and restore from a backup in case there is corruption, preferably giving yourself a larger disk for your blobstore before restarting.
RE "what happens if I completety remove all the data in the directory blobs/default/content": That is in effect removing all data from NXRM in the default blobstore. You will have no components if you do that.