what is roughly the split of rides between Uber Pool, Uber X and Uber Black?

This would vary wildly between markets.  For example in Cleveland, OH market, we very thankfully do not offer Uber pool.  The Cleveland market offers X, XL ,Select and Black.  The southern Cleveland market area offers only X and XL.  And yet there are markets in other areas that have additional services you didn't mention (and probably aren't aware of) such as UberSUV, Uber with child seat, Uber for handicaps and several other unique services.  

Uber has never released a break out nor do I anticipate they will anytime soon since any report would be misleading because of the variation within market areas.  BUT, with that said, in our market area (remembering that we do not offer Pool) UberX is by far the most popular service and since I also drive XL, I would have to say that at least 90% of requests during the week are for X with the XL option peaking on weekends with groups heading to and from an evening with friends.

Oooh, I did some math earlier this year. My car qualifies for UberSelect in my city. It's basically the equivalent of UberBlack, except my car isn't and doesn't have to be black.

As many suspect, Pool rides are still not that often, but considering it used to be 0% last year, it's definitely on the rise. I am actually surprised my UberSelect is so low as I had high hopes for making increased revenue than others, but I figured it allows me to keep busy and score higher ratings due to a nicer car.

UberX - 75% of all rides

UberPool - 10% of all rides

UberSelect - 15% of all rides

I did some additional analysis, but they were similar across the board.  For example, on average, my rides were 7 miles long and took 11 minutes. 

I qualify for UberXL, but 90% of my rides are UberX. Total bummer, as I thought a larger car would net me much more money. It stinks because I am incurring higher gas mileage to receive UberXs.

Then again, those 10% of the times, I make boosted revenue, so it definitely helps.

You should also keep in mind that this changes significantly from day to day. Say on the weekends, I get way many more XL requests. It also depends on where I am too.