Glassic cydia tweak makes your keyboard background transparent or translucent.
You can then change the background color of the keyboard, and even blur things behind the keyboard. Indecisive about what color to pick? Have Glassic change the keyboard color for each app, based on the app’s icon! So Youtube gets a red keyboard, Facebook a blue one, and the music app gets an orange keyboard.
If using one of the filters, the content behind the keyboard won’t animate after the keyboard appears. This is to save battery/cpu. However there is an option to refresh the filter every few seconds.
Configure options from Settings.