I'm not sure you'll be happy with any cold-bluing. Different products seem to work OK visually. Most require more than one application with a fine steel-wool buffing in between. But all of the cold-blue products are essentially dyes or markers that fill the pores, cover the surface, and not much else. They tend to be thin and easily worn off and offer none of the (somewhat limited) corrosion protection that true bluing provides.
Further, some cold-blue products match your existing bluing better than others so a bit of experimentation may be required.
Now if you're satisfied with just cosmetic coloring that will not wear as well as true bluing, most any product that matches your existing color will work for you. Be sure to clean and degrease thoroughly before attempting to cold-blue or risk mottled, blotchy appearance, and follow the directions exactly. Some folks use steaming hot water, Dawn (or other "degreasing") dish soap. I've used both dish soap and automotive degreaser with equal results.
If you want something more durable, you should consider either rebluing or an application product like Duracoat. Many people like to do these as DIY products and they are, supposedly, more friendly to DIYers than Parkerizing. However, most gun smiths will be able to do one or both, typically at very reasonable prices.
Peace favor your sword,