Miko has fallen, but she is not yet a Blackguard.
Personally, I think that during the fight with Roy, Belkar, and Hinjo she will realise her paladin powers are gone. Rather than think she is being punished for breaking her code, I think she will turn on the gods, believing they too are in the wrong. I think she will survive the fight, and later on she will appear again - likely as a blackguard. She will not be serving Xykon, however, as she believes him to be evil and thus one of her (several) foes. Instead, I think she will be fighting on her own.
Also, as I recall, didn't Belkar learn he might kill Miko's mount? I don't think it was specified that the mount would be the same one that Miko currently (or, rather, formerly) rode. I think instead that Belkar will slay Miko's next - fiendish - mount.
Note that while she stated to Shojo, etc that she had prayed to have the betrayers revealed, the flashback shows her praying to "see through their lies to the truth." She equated lying with betrayal. So while the deities granted the letter of her request (arriving just in time to learn of the truth), she misinterpreted this to mean those lying were those betraying - due to her own personal view.
In any case, this strip had several great lines, great action and drama, and an excellent cliff hanger. I look forward to the next strip.