We present ALMA 870 mu m and JCMT/SCUBA2 850 mu m dust continuum observations of a sample of optically dark and strongly lensed galaxies in cluster fields. The ALMA and SCUBA2 observations reach a median rms of similar to 0.11 mJy and 0.44 mJy, respectively, with the latter close to the confusion limit of the data at 850 mu m. This represents one of the most sensitive searches for dust emission in optically dark galaxies. We detect the dust emission in 12 out of 15 galaxies at >3.8 sigma, corresponding to a detection rate of 80%. Thanks to the gravitational lensing, we reach a deeper limiting flux than previous surveys in blank fields by a factor of similar to 3. We estimate delensed infrared luminosities in the range 2.9 x 10(11)-4.9 x 10(12) L-circle dot, which correspond to dust-obscured star formation rates of similar to 30-520 M-circle dot yr(-1). Stellar population fits to the optical-to-NIR photometric data yield a median redshift z = 4.26 and delensed stellar mass 6.0 x 10(10) M-circle dot. They contribute a lensing-corrected star formation rate density at least an order of magnitude higher than that of equivalently massive UV-selected galaxies at z > 3. The results suggest that there is a missing population of massive star-forming galaxies in the early Universe, which may dominate the SFR density at the massive end (M-* > 10(10.3) M-circle dot). Five optically dark galaxies are located within r < 50 '' in one cluster field, representing a potential overdensity structure that has a physical origin at a confidence level >99.974% from Poisson statistics. Follow-up spectroscopic observations with ALMA and/or JWST are crucial to confirm whether it is associated with a protocluster at similar redshifts.