simple help needed in photoshop

[TwoJ's]

i am new at photoshopping and i want to isolate one color and make the rest black and white in a photo i have. it has very deep blues in it and i want them to be over the rest b&w. what is this called?
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[straight6pwr]

theres not a specific tool for that as far as i know, youll just have to isolate all the blue manually.