I have eaten blackberry leaves a number of times. Despite them being thorny..
Pros:
~Rubus is an almost evergreen genus of plants. Therefore, these plants are a good winter crop, when other leaves are hardly available. Rubus laciniatus is in fact an evergreen species of this genus.
~Easily accessible.
~Perennial and by many considered invasive.
~The leaves are not bitter.
Cons:
Thorns. Young leaves are without them. Boiling softens the thorns, rendering them not harmful. Subsequent blending resolves the issue of thorns altogether.
One of my recipes which includes blackberry leaves is for a jam made of blackberry leaves and sultanas.
Ingredients for 1 serving:
~Blackberry leaves
~63 gram of sultanas or raisins [100mL]*
~90-215 mL of water [depends what consistency you prefer]
~a pinch of salt
Microwave on high-power setting until boiling (3-5 minutes). Blend thoroughly. Microwave again for 15-60 seconds.
*the density of sultanas is about 62.5 g/dL