The Physics Education Group at the University of Washington is examining student understanding of blackbody radiation. Results from interviews and questions administered in sophomore and upper-division courses indicate that traditional instruction on blackbody radiation often does not help students apply the concepts and mathematical formalism to real-world objects. We are developing an online homework that approaches blackbody radiation from a phenomenological viewpoint, rather than from an idealized formalism. Initial use suggests that this homework helps students understand, for example, how the spectrum of an incandescent light bulb changes with temperature. Moreover, students who have worked through the homework also seem able to provide more robust answers during interviews than students who have not. However, we find that students continue to struggle with the concept of blackbody radiation. Additional research is needed to be able to design more effective instructional materials.