Quote from someone else on reddit:

If say for example a hero with with a skill or item based critical strike chance attacks an enemy target affected by the Bloodthorn debuff they get a 100% crit chance for a 145% dmg multiplier every attack but they separately roll the dice/RNG for their built in crit so for example Phantom Assassin has 15% chance for a 230% / 340% / 450% damage multiplied crit at level 1/2/3.

If Phantom Assassin rolls her crit successfully, her crit overides the crit with the smaller damage multiplier completely so PA will do her 230% / 340% / 450% damage but no extra damage on top of that from the bloodthorne mutliplier.

If Phantom Assassin holds a Daedelus and doesn't get her Crit from her passive Ultimate ability maybe the 20% chance for 175% will come up and override Bloodthorns 145%.

All three could come up on the same dice roll but the lower damage multiplier crits just get overrided by the higher ones. You'll never get a critical on top of a critical for Phantom Assassin 450% multiplied again by the Bloodthorn or Daedalus critical multiplier because only the best available critical that rolls successfully is used.

Hope that makes sense.

I get daedalus on CK sometimes when I'm feeling clownly in pub games, and whenever the skill doesn't proc for a crit it then checks the item (daedalus) for a crit. Bloodthorn is checked last. You can never "crit on a crit," though, only one crit can proc at a time.