That little thing? Man I was expecting a long gouge down the length of the slide or something. I think the cold blue trick will work if you use care. Especially since it is so small. Of course you might see it on close examination, but do the blue, then put a thin film of oil over it and you won't ever see the glint of bare steel from any reasonable distance, and you might have to point it out to others. Now if it was a big round bare spot, then you would see the difference in the blue alot easier, but heck, that should be a cinch to fix. PS I fired the baby I once had and found it to be reliable and not bad to shoot. Only that there is not much there to hold onto. I got mine for like 300 and sold it for not much more. It had a place near the muzzle of the slide that was bare steel and had tiny rust pits. As I recall, the rest of the gun was perfect. I cleaned up that small spot and cold blued it too. Really not very noticeable and could pass the gunshow eye check unless you looked really really close. I was happy with the fix. Yours should look better.
s Instant Gun Blue. I've used this product on old beat up, rusty shotguns with exceptional reslut.