I don't think that is. Black TI is a ceramic plating and gun metal is metal plating. There is enough difference in the color that people would notice. I personally think Black TI has a considerably more flat look and gun metal a brighter look. BTW I can get all series kits from the same manufacturer in either finish if I want them.There are several that offer both. I think you can easily find Sierra Style kits offering both.
I have read that it is a Black Nickel alloy (it is not enamel)but have never asked my supplier about the formula. The plating process is probably the same as for most finishes. My supplier says they use the same process for all finishes except for the Titaniums, Rhodium and the enamels and some of their antique finishes.
So same manufacture, same kit offered in Black Ti AND gun metal? With sierra's there are more than one manufacture of them and one may call it 'black Ti' while another may call it 'gun metal'