  Polarized emission from aligned dust is a crucial tool for studies of
magnetism in the ISM and a troublesome contaminant for studies of CMB
polarization. In each case, an understanding of the significance of the
polarization signal requires well-calibrated physical models of dust grains.
Despite decades of progress in theory and observation, polarized dust models
remain largely underconstrained. During its 2012 flight, the balloon-borne
telescope BLASTPol obtained simultaneous broad-band polarimetric maps of a
translucent molecular cloud at 250, 350, and 500 microns. Combining these data
with polarimetry from the Planck 850 micron band, we have produced a
submillimeter polarization spectrum for a cloud of this type for the first
time. We find the polarization degree to be largely constant across the four
bands. This result introduces a new observable with the potential to place
strong empirical constraints on ISM dust polarization models in a previously
inaccessible density regime. Comparing with models by Draine and Fraisse
(2009), our result disfavors two of their models for which all polarization
arises due only to aligned silicate grains. By creating simple models for
polarized emission in a translucent cloud, we verify that extinction within the
cloud should have only a small effect on the polarization spectrum shape
compared to the diffuse ISM. Thus we expect the measured polarization spectrum
to be a valid check on diffuse ISM dust models. The general flatness of the
observed polarization spectrum suggests a challenge to models where temperature
and alignment degree are strongly correlated across major dust components.
