Black/copper stain on silver coin

Hi, I am very new to collecting and came across this coin at a vendor booth in the street.  It's a 1929 .500 silver coin from Brazil.  On the reverse, there is this crescent-shaped black spot.  I used the foil and baking soda electrolysis method to try to clean it up, and it turned a very copper color, but reverted to black in a couple of days.  What would cause that?  

I assume .500 silver wouldn't just be a silver-clad copper coin, right?  I did pass a strong magnet over it and I get the Eddy current resistance.  Also, I don't think this coin is valuable enough to fake.  

Any insight is appreciated, thanks.

These coins were heavily counterfeited at the time, maybe you can check the weight if matches.  

I have came across lots of those fakes, sometimes they get yellowish, sometimes they are really bad made sometimes not.

Btw it is a cheap coin you can replace with a better one.

Bem vindo ao Numista!

Geison

These coins were heavily counterfeited at the time, maybe you can check the weight if matches.  

 

Not what I expected to hear, but good to know, thanks.  The guy I bought it from didn't really know what he had.  He was selling a lot of tchotchke and just happened to have a stack of coins.  He has to consult the internet to see what he would sell them for (I had to talk him way down).  

The catalog lists it as an 8g coin.  On my ghetto kitchen scale it comes in at 7.9g.  I have another example from 1924 that comes in at 7.8g, which I bought from someone a little more knowledgeable.