Many sources (1, 2, 3, 4, 5, 6, for a start) suggest the word "fluke" has mostly positive connotations when used in the sense of "accident." That is, "a fluke" properly describes a lucky accident, not just any accident.
However, I sometimes hear this word used to refer to negative and to unmarked accidents. Is such usage, strictly speaking, correct?
To me, the best proof in support would be an example of the broader use by a first-rate writer.
Best Answer
The OP said:
Here is an example of fluke being used, first as an unlucky accident, and second, as a lucky accident. The quotation is perhaps too long, but it illustrates that the first speaker used fluke as unlucky, and the second as lucky. From E. Phillips Oppenheim, The Vanished Messenger
As for the neutral meaning of fluke, I cite from The Free Dictionary
(There are other dictionary definitions of fluke that support a neutral or negative meaning, but this answer is too long already.)