BB, Not my recipe (and not my comments) but might be at help...
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Syrup from blackberries.

blackberries
sugar
water

Fruits should be sound but fully ripe. Extract the juice by putting fruit in a
bowl, adding 150 ml water to 1.3 kg fruit, and place this over a pan of
simmering water for 2 to 3 hours. Strain through 3 to 4 thicknesses of muslin (I
always use an old dish wiping cloth). It will require 1 to 2 hours. The fruit
can be gently pressed until all liquid is released. (Do not press to much, for
the liquid will not be clear then). Measure juice, allow 325 g sugar to each 600
ml and place in a large pan. (Optional for flavoring, a few cloves or a piece of
cinnamon stick tied in muslin can be added). Heat to dissolve sugar. Do not
boil.

You can store this syrup, but then you have to sterilise it. Pour into sterile
bottles to within 4 cm of top. Place on screw tops, not too tightly. To
sterilise. Place a wire rack or false bottom in a deep vessel. Place the bottles
on this and fill with cold water up to neck of bottle. Bring to simmering, (180
degr.F, of 82 degr.C) in one hour and maintain for 20 minutes. Remove, screw
down tops, label and store in dark, cool place.

This is a recipe from a cookbook based on a television series in England. I make
this every summer from any kind of fruit, such as peaches and red currants, or
plain redcurrants, blackberries, but it never comes to storage.
Good luck, Tineke