My black berries wont bloom or fruit help?

When did you plant them?  If you just put them in this year, you won't see any fruit yet; blackberries do not fruit the first year.  Just keep them watered and apply a balanced fertilizer in spring and fall, so they can develop a strong, healthy root system.  Don't cut the canes back this winter!  The canes that grow this year will bloom and bear fruit next spring.  After the spring harvest, cut back all the canes that bore fruit; they are branched and will die.  The new canes that come out each year will be the ones that bear fruit the following year.  Taking good care of them now will get you a reasonable amount of fruit next spring, and a bountiful harvest the year after that. 

Three years ago I planted some blackberries in my garden that I had gotten from a nursery.  So far they seem to grow pretty good but have not put out any blossoms.  The raspberries growing twenty feet away are doing fine.  What can I try in order to get my blackberries to bare fruit?