Apple trees are considered sensitive to the toxic effects of juglone, a chemical that accumulates in the soil beneath black walnuts as it is released in small amounts by living roots and by dead roots as they decompose. It also gets into the soil from decaying black walnut hulls.  Apple trees should be located at least 35 feet from the black walnut tree's drip line.

Pear trees are considered resistant to juglone.  However, you probably should locate them well outside the drip line of the black walnut to reduce any competition for water and nutrients.