Cold blues are for touch ups only, and even there most are pretty poor. Brownell's Oxpho-Blue is an exception for cold blues, does a good job on touch ups, and if you prep well you can blue a large piece.
Best blue finish that isn't a hot salt blueing is Blue Wonder, a photo chemical reaction. Looks like a blue, but is very tough and corrosion resistant. I've done entire barrels and slides with it, and it works well.
Actually, if you are trying to refinish a muzzleloader, why not consider browning? Birchwood Casey's Plum Brown is not a bad product, much better than their "blues".