It looks like an old leaf which needs to be discarded.

But the story is different if the entire plant looks like that.

Please post an image of the plant, itself.

Here are some pics. Thanks!

We had a lot of this last year and earlier. Here is the fact sheet on what looks similar. http://www.extension.umn.edu/garden/yard-garden/vegetables/basil-downy-mildew/

Thanks. It is not Downy Mildew though. I had it last year.

Ok. If in doubt it is usually recommended to test the actual plant for exact disease, maybe you have already done that? We have a diagnostic disease place through the U of MN here, Here is one from Cornell http://plantclinic.cornell.edu/ I will send pics to our master gardener group as well to see if any comments come back for you. Good luck in identifying so you know how to treat it.

Thanks, I appreciate your help. I have contacted our local Master Gardener program at Purdue and they recommended that also. It actually looks like it has cleared itself up. I have been tarping them to avoid overwatering from our excessive rains and was able to give everything a good fertilization. They have greened up nicely and I have not seen as much of the black spotting. I am wondering if I might have burned the leaves with the fungicide I used and the problem was nutritional. I used the 5-1-1 mix from the container forum so it is very fast draining but it does not offer much nutrition. From what I have gathered it was created this way so you can control the nutrition. I was all for that until the rains came! I am adding Osmocote to each container to resolve this. I know herbs don't need a lot of fertilization but I think with this mix in particular, and overall container mixes, you need to give them a bit more than normal.