Pretty hard to tell what you mean from that picture; the bolt handle doesn't look blued.
If you just mean the texture, that's not from bluing, it's from surface prep prior to bluing. Bead blast, soda blast, acid etch, abrasive tumbled, etc are all options.
If you click on the photo and enlarge it you can see that the "collar" that the bolt handle is welded to is black and part way down the handle toward the knob is the same color/finish and gradually fades to the bare metal.
