I think it mostly depend on the style you write your unit tests. From the perspective of 'Uncle Bob' and Kent Beck and their way of teaching, it is more of a black box testing strategy because of the 'write your test first' approach. While from the perspective of the Steve Freeman, its all about code coverage. So its most of awhite box testing strategy.

These two type of unit testing styles are known as 'Detroit school' aka Classicists, and 'London School' aka Mockists. Search for 'Detroit vs London schools' and you can find many articles comparing each approach.