I tried to burn a synthetic carpet about 30 years ago, which was about 5 years before I learned through work that the materials had flame retardants in the formulation. Fast forward a few more years and those same flame retardants were reformulated because of EPA bans. Those chemicals were found in the environment including wild life and breast milk. I’m not a fear monger and I continue to ingest chemicals, but be advised that your blackberry fruits might not be great for you at some intake amount.