As with any good folk song, there are plenty of subtleties and ambiguities. If you wanted to, you could make a case for the song even suggesting that the master's death was in fact ENGINEERED by the servant.

The first verse describes a bunch of menial stuff the slave had to do when he was a kid, stuff the master could perfectly well do himself. Basically, the child has to all but wipe his master's arse. Not very dignified.:


When I was young I used to wait
On master and hand him his plate
Pass him the bottle when he got dry
And brush away the blue-tail fly

In the second verse, we see how the master is in fact dependant on the slave for his safety. A vulnerability is revealed, and revealed rather coyly in the faux-genteel "the pony being rather shy" line:

When he would ride in the afternoon
I'd follow him with my hickory broom
The pony being rather shy
When bitten by the blue-tail fly

Note the "chanced" in the third verse: the slave's testimony is very careful to emphasise contingency in his account (he'd be executed if there was any suggestion of negligence on his part, let alone - perish the thought - actively having engineered his master's death)

One day he rode around the farm
Flies so numerous that they did swarm
One chanced to bite him on the thigh
The devil take the blue-tail fly


Note "the jury wondered why" and the reported "the verdict was". Again, a reminder that there's real jeopardy here. But also that, if the jury wondered why, so might an attentive listener to the song:

Well the pony jumped, he start, he pitch
He threw my master in the ditch
He died and the jury wondered why
The verdict was the blue-tail fly

Chorus

Last verse: that master has literally been knocked down off his high horse to lying "beneath" the tree. Bit of irony there, considering how the slave used to water him when he was dry in the first verse; now that won't be a problem. The word lie crops up twice in that verse. (One might ponder the words "I'm forced to lie" here!).

The epitaph is another kind of testimony (a verdict set in stone): it was definitely the blue-tailed fly's fault. No question. Oh yes, definitely the blue-tailed fly. Just look at the epitaph. Just hear the jury's verdict. Nobody could possibly ever suggest that it was anything other than that pesky blue-tailed fly being responsible for the death of that guy. Oh no.

Not for nothing does fly rhyme with sly...

Now he lies beneath the 'simmon tree
His epitaph is there to see
Beneath this stone I'm forced to lie
The victim of the blue-tail fly