Question

Slopes are historically favorable for fruit trees because cold air drains down to lower ground. What other plantings do you have on the black clay & how are they doing? If the black clay is preventing water from percolating to the roots of fruit trees, you might want to become familiar with an information-rich Permaculture website > http://permaculture.org.au/ Permaculture teaches making catchments - using earth & other natural materials - to slow & redirect water runoff. They are usually called "gabons," & they vary depending upon the local circumstances. I had gabons made on sloping land after a wildfire burned over our property - to prevent erosion when winter snows melted off. They were a narrow trench dug with a small backhoe bucket & filled with large rocks, in arcs across the hillside & surprisingly inconpicuous & effective. PRI shows various gabons made without machinery also - build to fit! :-)

I hope my meaning above is clear - if your problem is the impermeable clay, you could create gabons upslope from your fruit trees to let water percolate down to them.

Your soil needs compost... lots of it. Eventually it will become more workable and hold moisture. Permaculture offers several solution to run-off problems. Read Toby Hemenway's book.