I believe it was almost certainly lightning. The black mark is a 'burn', from it. As it would likely have stopped her heart instantly, it should have been a quick death, thus no signs of thrashing(mercifully...)
When my uncle was raising QH's, we found a foal death out in the(huge!) pasture. He had that black marking over his poll, no other sign of injury/trauma. The dam was uninjured. This was rolling county, not a tree in sight, nor any fence nearby. It was unquestionably a death by lightning.
A very sad thing; my condolences...however, these things do occasionally happen.
Margo