It is only natural that someone of culture/faith X will seek religious specialists of culture/faith X despite being faced with black magic originating from culture/faith Y. This decision is always made out of cultural/religious familiarity and personal convenience. If the power of your religious specialists is sufficiently strong, it really does not matter that the black magic you are facing originates from a different faith/culture.
I'll give you a real-life example.
Pregnant couple (we'll call them Steve and Jenny) came to my teacher's place to consult Tua Ya Pek. I was there to help clerk their case. Jenny had been dreaming of a sinister old Malay-speaking Nyonya-ish woman who told her that she would take her baby and eat it alive (obviously a pontianak). Steve told her not to think too much of this, until Jenny started bleeding from the birth canal. Doctors did scans and all that and said that things were fine, but they were scared stiff, the baby being very precious due to them having tried for a long time, and begged Tua Ya Pek to help.
Tua Ya Pek gave them a talisman that Jenny had to wear around her pregnant belly, and told them that the next time the pontianak came by, "She's gonna wish she never died."
Few nights later, Jenny dreamt of the pontianak trying to physically attack her, but the vampiress burst into flames the moment she tried to lay a hand on Jenny. Jenny said she could hear the pontianak screaming in pain even after she woke up in a cold sweat. Steve didn't hear any screaming, but he heard a loud flapping noise outside the bedroom window like some sort of large bird was trying to get in. All they could do was hold each other tight until the noises eventually stopped.
No more trouble at all after that. Baby Rachel was born smoothly, is now a talented teenager and has totally no clue about what threatened her in the past.
Very fascinating sharing.
It renews our faith in the deities. More please....