It's just a backslash in debian/control. Patch attached.
diff -u mumps-4.8.4.dfsg/debian/control mumps-4.8.4.dfsg/debian/control
--- mumps-4.8.4.dfsg/debian/control
+++ mumps-4.8.4.dfsg/debian/control
@@ -3,7 +3,7 @@
Priority: extra
Maintainer: Adam C. Powell, IV <hazel...@debian.org>
Standards-Version: 3.8.1
-Build-Depends: debhelper (>= 3.0), quilt, gfortran, libscalapack-mpi-dev, \
+Build-Depends: debhelper (>= 3.0), quilt, gfortran, libscalapack-mpi-dev,
libblacs-mpi-dev, libblas-dev | libblas-3gf.so, mpi-default-dev
Homepage: http://mumps.enseeiht.fr/
