In quantum computation it is often assumed that if $f$ denotes some (classical) Boolean circuit $\{0,1\}^n \rightarrow \{0,1\}$, then a quantum circuit can have oracle access to $f$, that is the quantum algorithm can query in superposition via,
$$\lvert x, y\rangle\ \mapsto\ \lvert x, f(x)\oplus y\rangle$$
What evidence is there that this (i.e. querying in superposition) is indeed a physically reasonable assumption? Given blackbox access to some $f$ how would one go about integrating this into a quantum circuit?