Add Blackberries to secondary fermenter as you rack the beer. This should help prevent contamination from the blackberries as they will have possible yeast/bacteria on them. I suggest freezing the blackberries beforehand for better juice extraction. If possible wash them before this to reduce any bacteria/yeast that may be present. spraying the fruit with properly diluted star san may also aid in this prevention. Note that contamination in a fruit beer is always a possibility, we will just try and minimize that risk as much as possible.