Yellow spot on limes

Hi My lime tree has finally managed to keep some fruit. I have about 6 limes on it now. However, 5 have a yellow spot - the size of a 50 cent piece on the blossom end. There is no visible poke, hole, bug, scab, scar anything, other than the color change. The fruit is still hard and small, size of hand ball. There is nothing to indicate disease or insect damage. I am just perplexed. I have been looking online for some info, but none of the pictures/descriptions I have seen resemble my problem. Any help is greatly appreciated.
AnaM

Here are a couple of pics to help...

One more...

Ana,
That looks like Sooty Mold on the leaves of photo #2. If it is Sooty Mold, it indicates the presence of insect pests. The yellowing with the black is spot could be Alternaria Brown Spot. The yellowing and black spot are symptoms of that disease. However, Florida has problems we don't have here in Texas. I would suggest you take a sample of the fruit and leaves to your local country Cooperative Extension agent. If he can't give you an answer, he has access to university researchers who will know the answer.
Veronica

Thank you for the information. Yes, i have sooty mold on it. It is tough here with all the humidity to effectively clean the tree. My orange had it for a few years, too, but this year it is much much happier. I will probably have better luck taking it to my county office. And I am fortunate that UCF is 20 minutes away that I can take it there, too. Thank you!