This study considers blind channel shortening to mitigate the effects of inter-block interference and multiple-access interference in both uplink and downlink multicarrier code-division multiple-access systems. We derive the blind channel shortening methods by extending the blind channel equalization method previously proposed by Djebbar, Abed-Meraim, and Djebbari. The basic idea is to restore, at the frequency-domain equalizer output, the orthogonality of spreading codes, which is destroyed when interference is present. We show that the extended methods can perfectly shorten the channels and that the resulting effective channel is identical to the predetermined target channel thus avoiding channel estimation. Computer simulation results illustrate the effectiveness of the proposed methods.