A quick initial point but I think it's wrong to characterise informal markets as being either 'under the counter' or 'black markets'.... (with apologies for dubious use of the term in this context)

An informal market can simply be bartering or exchange i.e. a 'market of exchange' based upon a mutual recognition of the others worth (what they bring to the table) that is not reliant upon the sale of goods or current market money values.

After all, George Ayittey's tone when he refers to informal markets is positive and there are more 'markets' than the 'free marketeers' would have us believe. ;)