The bitter flavor you tasted was most likely due to the beans being fresh from the roaster. After roasting beans out gas CO2 which is very acidic. As your beans were fresh from the roaster the CO2 was still in the beans, hence their bitter flavor. As the beans age they will gradually mellow and typically hit peak flavor in 5-10 days with some beans taking as long as 3 weeks. Store the beans in a airtight/one-way valve container/bag in a dark cool place and try tasting them on a daily basis to determine when they taste best to you and your brewing method.
Java "Enjoy the journey!" phile
PS Get rid of the popper's top and replace it with a chimney/can.