The word 'blár' can mean 'dark' or 'black'.

Bragi the old sung:

Rósta varð í ranni
Randvés höfuðniðja
þá er hrafnbláir hefnðu
harma Erps of barmar.

There was an uproar in the house of Randvér's
great kinsman when the raven-black brothers
of Erpr avenged their sorrow.

Ravens are certainly not 'blue' but the word
'blár' can be applied to them. This is a good
word to demonstrate a type of contraction.
When a word whose stem ends with 'á' should
get an a-ending it often doesn't.

Hrafn er blár.
Hrafnar eru bláir.
Óláfr sér blán hrafn. (contraction)
Óláfr sér hrafna blá. (contraction)

This is no longer true in the modern language;
we now write:

Ólafur sér bláan hrafn.
Ólafur sér bláa hrafna.

This is not surprising when you consider that in
Old Norse 'á' is simply a long 'a' and thus easily
assimilates it whereas in modern Icelandic 'á' and
'a' stand for different sounds altogether.

Kveðja,
Haukur

P.S. Hvárt er hrafn sá er Óláfr konungr sér hvítr?
Eigi er svá. Blár er hrafninn sem allir hrafnar.