Open fermentation

So I had been wondering for days why there was no action in my airlock, despite obvious yeast action, and finally just realized that one of the pieces of the airlock was missing... in essence it's been sitting exposed for days. It smells more or less like normal fermented wort (hard to tell with a stout) but the otherwise white foam had a dark section in the middle. Is there cause for concern? Would I be able to taste the contamination?

Only been a few days? No, I wouldn't worry about it. That thing has been pumping out CO2 faster than stuff can get in.