Pretty much what the title says. I didn't find online where Mythical Champions get extra bonuses from Legendary Blessings like how it normally works for when champs of a higher rarity use a lower Blessing. 

I'm sure it would be an excuse to make a $60-$100 Soul Shard too if you had Mythical Tier Blessings.

Also quick aside: Since the feedback on Primal Shards isn't going to get you to change the Drop Rates, it would be a good idea if you made Primal Shards capable of summoning some of the unobtainable Champions in the game like Ninja or getting duplicates of Mithrala or something along those lines.