Blank nodes (or anonymous nodes) are part of RDF and are not a problem at all. They are essential to build embedded resources. When the RDF graph moves physically (e.g. from triple store to triple store) they must get renumbered internally so the embedded resource is still valid at the other place, and can be continued to use in the new environment without internal graph node collisions.
These tasks are opaque to applications, they are part of RDF graph management of the RDF store implementor, and are not related or controllable by Bibframe.
All embedded resources can be turned into externally visible resources by assigning IRIs to them, if you want to avoid embedded resources. The result is often too verbose and does not look very concise, and in most cases, there is no requirement of doing that.