A blind study means don't tell the DNA testing company which submissions are for real cases and which are for testing. A double-blid study means don't tell the police submitting it either (to avoid giving subtle clues to the testing company, like calling to follow up on tests but not real cases, or vice versa).
If they (the testing company and the police) are willing to conduct a double-bind study, then its results are a reliable way to assess the technology. If they are NOT willing to conduct a double-blind study then we should ask why not. The answer is almost certainly because they are afraid of what the results might show.