To my knowledge there is not a way blur an image natively within the sdk, you can use the maskGroup feature but this is not available on all devices (https://sdkdocs.roku.com/display/sdkdoc/MaskGroup
One way you can accomplish this is simply create an image with some opacity in photoshop and then place it overtop of the image you want to blur. so for example if you have say a modal that pops up over top of your main screen to display information about the content, you can set the modal size to be full screen and then set the modal background image to your semi-transparent gaussian-blur image. This method should work on all devices.
as a test i just took this image https://wallpaperlayer.com/img/2015/6/gaussian-blur-wallpaper-3240-3444-hd-wallpapers.jpg from a google search, set its opacity to 90% in photoshop and then applied it over top of an image and achieved an effect similar to the gaussian blur effect.
Hope this helps! have a good one.