I have tried that once, but on its own, it just shrivels. I cannot say it had that much effect on the raspberries.
I have very successfully mixed comfrey with grass cuttings and either horse manure (well rotted) or moss peat (a way to finish a half empty bag) when hilling up potatoes. Whether the pH of that mixture would be too alkaline for raspberries, I do not know…..
I use it around my Tomato plants. I find it decomposes very quickly on its own so I mix it with Leafmould thereby saving me the trouble of having to renew the mulch so often.