What do opaque rims on biotite in a volcanic rock mean?
There are several options.


During decompression, biotite is no longer stable and the hydrogen volatilises, leaving behind fine-grained intergrowth of oxide and silicate minerals.
Oxidation of the rims during exposure of the lava to the atmosphere. Higher Fe3+ contents would make the biotite darker.


It is impossible to tell without a detailed electron microscopy analysis. However, my guess it that it is option 1.