Might be bad seeds because the Blackberry is supposed to show color throughout the entire flowering cycle. HIGHTIMES.COM > PURPLE POT

"This brings us to the third and final degree of purpledom, the Blackberry. This rare phenotype exhibits a deep, dark purple bordering on black on all its calyxes (even nodal), under all circumstances except extreme high temperature, from the start of flowering through harvest. The Blackberry??s taste is as unique as its appearance, with scents all in the ??high-note? range; tip of the tongue, if you will. There are no earthy, musky tones at all, but rather, elements of violets and opium with a slight antiseptic note. Even the trichomes of the Blackberry are purple, which makes extracting bubble hash a rewarding process, as both the water and the hash end up purple. (photos Blackberry nugs, BlackBerry bubble hash, Blackberry bubble H2O) "

"Another factor that can cause purpling is nutrient deficiency, generally phosphorus. Although these stunted plants may bedazzle the novice, they are typically quite distinguishable from naturally occurring anthocyanin expression, due to the other visible adverse side effects of nutrient deficiency, such as leaf and bud malformation and low calyx-to-leaf ratios"