Yes it scales with spirit, but it also becomes "worse" when you get T2 3set since the casting regen will become closer to the non-casting regen so that when the trinket procs you "gain" less mana.
For example let's say you have 100 mana per tick when not casting, and ALL of it is from spirit (about 308 Spirit).
If you have no talents and no T2 you "gain" 100 mana per tick when this tricket procs since your regen when casting is 0, making it a total of 600 mana.
With talents this makes it so that you regen 15 mana per tick when casting, so when the trinket procs you "gain" a total of 510 mana.
With talents + T2 3set bonus you would regen 30 mana per tick when casting, so when the trinket procs you "gain" a total of 420 mana.
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When you gain more than 92 mana per tick from the proc (for example you have 8 mana per tick when casting and 100 when not casting), that is when it becomes better than mindtap talisman.
Since you would want to have 3set T2 and the 15% regen talent, your casting regen will always be at least 30% of your non-casting regen. The breaking point would be if you have 132 mana per tick when not casting and 39.6 per tick when casting from SPIRIT, that is when it becomes better than mindtap talisman.
This should occur when you have about 406 or 407 spirit if my calculations are correct.
So if you have over 406 spirit then Darkmoon Card: Blue Dragon should give you more mana than mindtap talisman, (THIS IS ASSUMING YOU CAST 2.5 SEC SPELLS WITHOUT INTERRUPTIONS FOR THE DURATION OF A FIGHT AND THE TRINKET WILL PROC ON EVERY 50TH SPELLCAST, ASSUMING THE FIGHT IS AT LEAST 125SECONDS LONG) however it is still very unreliable and I wouldn't even use it then.
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The reason why this trinket used to be good it that it used to stack with both T2 3set and the talent so when it procced you got 130% mana regen - but that's not how it works on this server.