Tbh it doesn't even matter since the number of Swift Parry and Deflection artifacts you can craft will still be limited by how many Forge Hammers you have. 

Actually currently it does matter because I already have a set of usable Deflection set and I still don't have a full usable set of swift parry.

And believe it or not, currently the limiting factor is not the hammer but the legendary Soulstone, in which on the other side I have quite a pile of the bloodstone.

And it's not just about the Swift parry, the same thing also effect the forging of the Resilience set.

So again, why is the soulstone is significantly more rare than the bloodstone?

I just checked mine- I currently have:

677 Legendary Soulstones, 562 Epic Soulstones

690 Legendary Bloodstones, 537 Epic Bloodstones

I don't think there is a disparity in the first place, you may have more of the other at any given point in time but it doesn't mean that Soulstones are significantly rarer than Bloodstones. I bet you'd find other people who experience the opposite.

I realise my disparity when I try to sell off my rare stones (by forging them at the 3-4 * tier Resilience/ Perception)

I noticed my rare soulstone is only at around 250 someth anding my bloodstone is at 820 something.

Currently I have 14 legendary bloodstone and 6 legendary soulstone. BUT 3 days ago I forge 3 or 4 of the 5-6* Perception and only 1 of the 5-6* Resilience.

There is definitely a disparity on my account, but I will track it more definitively after today.