Kdenlive/Manual/Effects/Blur and hide/Box Blur
This is the Boxblur MLT filter.
Separate horizontal and vertical blur.
The unit for the boxblur "hori" and "vert" parameters is "pixels". For each pixel in the image, the parameters indicate what pixels will be mixed with it in order to create the blur effect. For example, if you set "hori" to 5, then each pixel will be blurred with the sample that is 5 pixels away on each side.
Not 100% sure if this sample is Box Blur or Square Blur. The caption on this sample was translated from Spanish. Original Spanish was "Difuminar en cuadro".