You’re right hjbau and your last sentence just reminded me of something. Snow did have that fairy dust and she wanted to use it on Regina. She might’ve tried to kill her before … On the other hand, Snow wanted to leave the EQ’s kingdom and go somewhere far away. That implies, she doesn’t want to do anything and only has the dust for an emergency.
Back to the original questions:
I think those black guards/the EQ’s guards are either on her side because they like her ways, don’t care, are afraid or have lost their hearts to her (literally). The other guards are still loyal to Snow. THAT is why she needs the huntsmen. She can’t take her own guards, not only because they’re loyal to Snow, but because even if she did use a soldier on her side, the others would know, she sent him. That’s my two pence.