[dice]

If the roots are not taking in enough calcium, the fruit will
get BER. They could not take in enough because there is not
enough in the soil, because the soil is too dry (the calcium
has to dissolve in water for the plant to take it up), because
the root ball is too small for the plant, because there are high
levels of other minerals that interfere with calcium uptake,
because the soil is too wet (root hair die off), etc.

Unfortunately, while this condition (actual effective calcium
defiency in the soil or growth media) will cause BER, that is
not the only cause. Anything that interferes with the transport
of calcium from the roots to the fruit will also cause BER (usually
this means variable weather and soil moisture levels). Rapid
vegetative growth can also caused increased BER by using up
too much of the calcium available the plant for leaves and stems.
Research has also reported higher BER rates when using
fertilizers where most of the nitrogen content was some form
of ammonia. (This probably only applies to ammonium that is
added midseason, when fruit are developing. Ammonium
added in spring gets converted to nitrates by bacteria
within a couple of weeks.)

(I do not know of any research testing BER raters with urea
fertilizers.)

This chart from TotalGro shows what nutrients in excess can
interfere with calcium uptake. (Magnesium and ammonium
seem to be the most glaring offenders. Epsom salts, magnesium
sulfate, would seem to not be recommended for this condition.)