BBM has just been updated on all three platforms it is currently available on - BlackBerry, Android and iOS. With it came lots a changes and additions, mostly on the non-BlackBerry platforms, like BBM Channels and BBM Voice. One thing that was also added a was an action bar that sits on top on the virtual keyboard when you are in a chat conversation window. This action bar has always been present on physical keyboard devices but for full touchscreen users, you always had to hide the keyboard in order to see more options. So to make it easier for users, the action bar, complete with a 'Back' option is now present. You will also find this present in the the updated iOS and Android BBM version.
While this is an option welcomed by a lot people, there are others who may feel it takes way from screen real estate. If you're in that boat, fear not, you can hide the action bar. This can be done on BlackBerry devices as well as BBM on Android and iOS. Though it must be noted that while you can toggle this option on the BlackBerry Q10 and Q5, it doesn't make the action bar disappear. It looks to be a permanent feature on the physical keyboard devices.
When the toggle is set to off, you will no longer see the action bar when the virtual keyboard is showing. You will need to minimize the keyboard in order to access those items in the action bar.