If you pack a scoped package using npm pack, the name of the tarball will be <scope>-<name>-<version>.tgz, e.g. npm pack @xmldom/xmldom results in the file xmldom-xmldom-0.8.6.tgz. npm pack also prints the filename to stdout.
Source: the npm documentation
When blerf tries to determine the path to a package tarball, it does not consider this. It simply does path.join(packagePath, packageJson.name + "-" + packageJson.version + ".tgz"), which results in path/to/@<scope>/<name>-<version>.tgz instead of path/to/<scope>-<name>-<version>.tgz.
For the above @xmldom/xmldom example that would be path/to/@xmldom/xmldom-0.8.6.tgz instead of path/to/xmldom-xmldom-0.8.6.tgz.
This of course results in all kinds of errors - mostly not finding files where it expects them.