Who has not reported to Lee, thus making him "blind" in enemy country?
The cavalry commander that Robert E. Lee is referring to is James Ewell Brown Stuart (Jeb Stuart). The battle that this happened during is the Battle of Gettysburg in the year 1863.
Jeb Stuart was very good at his job. Lee said the following of Stuart's capabilities:
[Stuart] is a rare man, wonderfully endowed by nature with the qualities necessary for an officer of light cavalry. ... Calm, firm, acute, active, and enterprising, I know no one more competent than he to estimate the occurrences before him at their true value. If you add to this army a real brigade of cavalry, you can find no better brigadier-general to command it.
As the American Civil war progressed, General Lee came to depend on Stuart's judgment and cavalry. Stuart would frequently go on reconnaissance missions for Lee in order to report on Union troop movements. Stuart was also given free reign to hamper and antagonize the Union as he saw fit. During two such reconnaissance sorties, Stuart led his men on a complete circumnavigation of the Union army. His men stole horses, wagons, food, supplies, etc. These daring exploits earned him fame across the confederacy. Unfortunately though, it often left him far out of communication with his commanding generals.
That's what happened at Gettysburg. Whether Stuart was going for a third circumnavigation or not is up for debate, but what is definite is that Stuart arrived late at Gettysburg. That left Lee without Stuart's highly mobile unit and intelligence gathering capabilities for far too long. Lee just didn't have a clear picture of what he was up against. That is why Lee felt like he was blind in enemy country.