It's always better to be safe then sorry and use a blow off tube.
As to flavors being scrubbed out. the yeast will eat most of the natural sugars that make blackberry sweet, so you will lose most if not all of the sweetness. If the wife want a sweeter brew, the best way to deal with this is to back sweeten with a sweetener that yeast can not ferment such as splenda. Rather than sweeten the whole batch, many people just sweeten selected bottles, or sweeten as they drink it. That way you don't accidentally over sweeten the whole batch, and can adjust each beer individually, or not at all to individual preferences.
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