The Blur effect is keyframable – at least in the latest Kdenlive versions (not available for Ubuntu 14.04, for instance): when you apply the effect, you will find, under the B with a cyan background, a tiny round icon with an ‘Add keyframes’ hint. Just click on it, and you’ll get two sliders to set the blur amount for the beginning and the end of the clip. You can also add new keyframes. Gaussian blur will provide an effect which is very close to the OP’s needs.
PS: The try was done with the former 16.12 Windows version. Just after, I upgraded to the latest 17.04 Windows version, and the background is no longer cyan but green (?) Maybe linked with the emergence of a French translation: I don’t like translations, because they are generally inaccurate, cause errors and even sometimes create bugs. At least this one did not correct the annoying bug of Kdenlive Windows not closing its processes when you close the program;)
P-PS: I restored the language to American English, and the background of the B is cyan again…