Bowden Tube Damage
Also see Bowden PTFE Tube
Gouge in the tube
As soon as the Bowden Tube Fitting grabs onto the Bowden Tube it will gouge into the tube. This is expected and not an issue until the gouging becomes worse. That can happen over time especially if the bowden tube is frequently removed.
Melting:
In this example picture the tube was not melted, the black color is from PLA residue. If you see a black end, do not assume the temperature was high enough to melt the tube. Maximum service temperature is 260°C. 3DKB recommends a maximum Nozzle Temperature of 240°C if the bowden tube extends into the hotend. For higher temperatures consider a All Metal Hotend. Rubbery State begins at 127°C but is unlikely to cause issues unless there is prolonged printing at higher temperatures. In the example picture above this wear damage occurred printing at 205°C and 1000 hours of use.