  The submillimeter opacity of dust in the diffuse Galactic interstellar medium
(ISM) has been quantified using a pixel-by-pixel correlation of images of
continuum emission with a proxy for column density. We used three BLAST bands
at 250, 350, and 500 \mu m and one IRAS at 100 \mu m. The proxy is the
near-infrared color excess, E(J-Ks), obtained from 2MASS. Based on observations
of stars, we show how well this color excess is correlated with the total
hydrogen column density for regions of moderate extinction. The ratio of
emission to column density, the emissivity, is then known from the
correlations, as a function of frequency. The spectral distribution of this
emissivity can be fit by a modified blackbody, whence the characteristic dust
temperature T and the desired opacity \sigma_e(1200) at 1200 GHz can be
obtained. We have analyzed 14 regions near the Galactic plane toward the Vela
molecular cloud, mostly selected to avoid regions of high column density (N_H >
10^{22} cm^-2) and small enough to ensure a uniform T. We find \sigma_e(1200)
is typically 2 to 4 x 10^{-25} cm^2/H and thus about 2 to 4 times larger than
the average value in the local high Galactic latitude diffuse atomic ISM. This
is strong evidence for grain evolution. There is a range in total power per H
nucleon absorbed (re-radiated) by the dust, reflecting changes in the
interstellar radiation field and/or the dust absorption opacity. These changes
affect the equilibrium T, which is typically 15 K, colder than at high
latitudes. Our analysis extends, to higher opacity and lower T, the trend of
increasing opacity with decreasing T that was found at high latitudes. The
recognition of changes in the emission opacity raises a cautionary flag because
all column densities deduced from dust emission maps, and the masses of compact
structures within them, depend inversely on the value adopted.
