This thesis experimentally investigates Bloch oscillations of a Rubidium-87 Bose-Einstein condensate in an disordered optical lattice. The disorder is created by superposing an additional, incommensurate optical lattice to the main lattice. Individually, collisional mean-field interactions between atoms as well as disorder both lead to a damping of the oscillations. However, we observe a competition between the two contributions, which results in an effective reduction of the disorder-induced damping due to mean-field interactions. This observation is consistent with an interaction-induced screening of the potential.