Some languages do have a middle voice in their grammars. Modern English doesn't.
That being said, your understanding of this intransitive use of "to soak" seems to be correct. The term middle voice is still used to explain the semantics of English clauses in which roles like agent and patient* cannot be clearly separated.
I walked to the store. / I walked myself to the store.
Blood soaked through my glove. / Blood soaked itself through my glove.
In the second sentence of each pair above, a clear grammatical separation exists between the semantic roles. However, the second sentences are questionable; they are not available in every dialect.** In the first sentences, the distinction is blurred. The subjects represent both that which performs the action and that which is transformed by the action.
I was walked to the store.
Blood was soaked through the glove.
These sentences have a passive voice construction. In the passive voice, the subject cannot be the agent. Someone or something other than myself walked me to the store. Someone or something other than the blood soaked it through the glove. It is this suggestion of an external, unmentioned agent that makes these passive-voice constructions seem intentional rather than spontaneous -- the agent (which must exist) may well have performed the action on purpose.
* I do not intend to distinguish between agent and actor or between patient and theme. Consider agent to mean agent or actor or perceiver or any other semantic role that suits the subject of an active-voice clause. Likewise, consider patient to represent any role that makes a sensible direct object.
** They happen to be available in my native Midwestern dialect. Even so, I'm more likely to use the first sentence in each pair, unless I have some specific reason to emphasize the lack of any other agent.