The Goddess Symbol and Purification.

When Yoshiro activates the symbol, it pulls defilement into it and transfers it into Yoshiro.

Her map symbol is a katashiro, designed to absorb impurities.

She stands atop the symbol as this is the defilement's final destination: her body.

It resembles the Kaleidoscope effect: also called Sun Halo or Moon Halo. (Yoshiro and Soh)

Sun Halo

Moon Halo

When you defeat Festering Seethes, Yoshiro pulls in all of the defilement and in doing so, purifies it.

When Yoshiro falls in battle, all of the impurity she holds within bursts forth from her in what appears to be an endless stream.

We(the screen) are also covered in impurity.

Soh's mask has the impurity pattern on it and we are led to believe the Maiden eventually becomes the next Messenger. 

I don't think the Maiden becomes the Messenger the way we think she does. It's better to say that the Maiden is no more and has fused with the Goddess entirely. There is only the Goddess who sends herself as the next Messenger.

Could Soh's mask have been clear originally but through fusing with these Maidens, it took on this pattern, symbolizing the Goddess steadily becoming cursed?

A little more on this symbol:

It's officially called 'Shrine Symbol' according to Steam.

In the middle is the sword that is associated with the Goddess, as seen here:

Yoshiro acts as an abode and shrine for the Goddess, as well as a katashiro. 

If you look at the symbol, then look at the following image, don't you think it's a visual representation of what Yoshiro is meant to do?