The Elder Scrolls V Skyrim – Purpose of the Black Soul Gem

At first, this seems pretty straightforward:

• Petty gems hold non-humanoid mobs, below level 4
• Lesser gems hold non-humanoid mobs, below level 16
• Common gems hold non-humanoid mobs, below level 28
• Greater gems hold non-humanoid mobs, below level 38
• Grand gems hold any non-humanoid mobs, regardless of level
• Black soul gems hold any mob—humanoid or non-humanoid—regardless of level

So if you want to use your grand soul gems efficiently, you'll want to reserve them for mobs level 39 or higher.

But determining the mob's level is the tricky part. You can use the console command getlevel.

However, you don't have to worry too much about that, especially for soul gems. Bethesda knew you wouldn't know what level mobs are, so Soul Trap will automatically use the smallest possible gem you have available.

So if you want to efficiently use your grand soul gems, always have a stockpile of the smaller gems handy.

According to the UESP wiki, the Black Star should only get filled with black (i.e. humanoid) souls, but due to a bug/glitch (as you said) it also accepts 'white' (i.e. non-humanoid) souls.