Are you referring to cold-bluing? It's good for touch-ups but it will never match the quality of hot-bluing. I've used Oxpho-Blue and Birchwood Casey Perma Blue. Both do a pretty good job touching up. I once tried to re-blue an older Marlin with 'em and the results were not great. Pass-able but not great. If it isn't a valuable piece, no big risk in experimenting.
"You haven't lived until you've died." - various