Clearly McCullough did her research, as did novelists such as Robert Graves, but a novelists nature is always to take that research and insert their own interpretations of it. Are they wrong? Not necessarily, but there is often not enough actual historical to support some of the details.
However, in this case, Plutarch provides some interesting information. He, unlike us unfortunately, would've also had access to Sulla's personal memoirs.
Plutarch provides a decent summary of the 'blotchy' skin condition in Life of Sulla ch 2
Describing his death, Plutarch goes on to explain the condition with worms that Kosmo mentioned. Life of Sulla ch. 36