Shouldn't Mana be a Copied Attribute?
When they say &quot;the value of X&quot; in rule 706.10, they mean specifically the value of a mana cost of X, the way it appears in, say, Hurricane. This X is an inherent part of a spell on the stack; as you cast a spell with X in its mana cost, you choose X at the same time you would for instance choose to pay for kickers and other alternate / optional additional costs, and the X is included in the converted mana cost as long as the spell is on the stack. As such, that value will be copied if the spell is copied.
However, the X in your Blessing is just another way of writing &quot;Distribute a number of +1/+1 counters among creatures you control, equal to the amount of snow mana used to pay for this particular spell.&quot; And if you copy a Blessing, no snow mana was used to pay for the copied spell. That X isn't part of the spell itself. It is only part of the effect that happens when the spell resolves.