You can't be proven wrong. A giant ball of milk, if compressed adequately, would form a black hole. It might seem reasonable to say that a black hole formed from a ball of milk "contains" that milk.
On the other hand, you could never see the milk inside it ever again, and, in physics, it's considered a faux pas to speak of what you cannot observe. While no one can disprove that your black hole has milk inside, you also lack the ability to prove it.
There is a famous theorem called the "no hair" theorem, which states that the only physically observable characteristics of a black hole are its mass, charge, and angular momentum. Black holes literally have no other properties besides those three quantities, no matter what kind of matter formed them.
You can think of the formation of a black hole -- the actual collapse -- as an event that destroys some of the characteristics of the collapsing matter. Analogously, if you were to heat any two substances up sufficiently -- milk and orange juice, say -- they would break down into protons and neutrons and electrons. You could not tell the two substances apart anymore except perhaps by counting the number of neutrons vs. protons, and even that knowledge is not sufficient to conclusively indentify one substance as milk and the other as orange juice.
In collapsing into a black hole, matter loses all its characteristics except mass, charge, and angular momentum. There is no way to tell a "milk" black hole from an "orange juice" black hole, even in principle.
- Warren
