I had heard this before, but I couldn't remember where, so I set out to investigate. I was surprised how little information I could glean on the subject from the Internet, and this led me to wonder how much truth there was in the story.
Wikipedia mentions it only in passing: "[after the Column of Victory,] the next point on the great axis, planted with trees in the position of troops, was the epic Roman style bridge." Further on: "the Column of Victory ... terminates a great avenue of elms leading to the palace, which were planted in the positions of Marlborough's troops at the Battle of Blenheim."
Wikipedia doesn't give a citation for either of these references – which may be significant.
A site celebrating the tricentenary of Capability Brown in 2016 (or a festival in his honour) has a detailed description of the landscaping carried out by Brown over a period of ten years, starting in 1763. But it doesn't mention trees in battle formation.
The only other reference I could find was on British History Online (BHO) – a resource for academics. Here I read that "The idea that the avenue's design represents the disposition of opposing armies at the battle of Blenheim seems to be a variant of late 18th–century allegations relating to the park as a whole or to Wise's plantations in Lower Park."
Not exactly a definitive confirmation!
The BHO page has a map of the park, with trees marked along the Great Avenue. It's easy to imagine the trees on either side representing opposing troops, but not being a military historian I can't comment on how realistic this might be. Although they are entirely symmetrical, which doesn't strike me as being very likely.
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