  The distance-duality relation (DDR) between the luminosity distance $D_L$ and
the angular diameter distance $D_A$ is viewed as a powerful tool for testing
for the opacity of the Universe, being independent of any cosmological model.
It was applied by many authors, who mostly confirm its validity and report a
negligible opacity of the Universe. Nevertheless, a thorough analysis reveals
that applying the DDR in cosmic opacity tests is tricky. Its applicability is
strongly limited because of a non-unique interpretation of the $D_L$ data in
terms of cosmic opacity and a rather low accuracy and deficient extent of
currently available $D_A$ data. Moreover, authors usually assume that cosmic
opacity is frequency independent and parametrize it in their tests by a
prescribed phenomenological function. In this way, they only prove that cosmic
opacity does not follow their assumptions. As a consequence, no convincing
evidence of transparency of the universe using the DDR has so far been
presented.
