Why can't I remove the blank default template in RHEV-M 3.0?

This is due to internal implementation concepts that are exposed to end users.

1. Each virtual machine has to be derived from a template, thus the need for a blank template.
2. All templates must be associated with a cluster, hence the default cluster was chosen to 'host' the blank template.

With that said, you have got to have at least one data center and one cluster in the system anyhow. So if the name 'default' does not meet your needs, you can just rename it. You can rename both the 'default' cluster and the 'default' data center and reuse them for your needs.