Why does the outlet pipe of my Beko washing machine get blocked up?

About every 3 months or so, the outlet pipe of my Beko washing machine gets so blocked up with a horrible black gunge that the machine cannot empty properly. I have to disconnect the rubber pipe from where it joins just above the sinks U-bend and clean out the blockage. I check the pump filter at the front of the machine and it is totally clean. What is going on please????