We present the results of variability power spectral density (PSD) analysis using multiwavelength radio to GeV 3-ray light curves covering timescales of decades/years to days/minutes for the blazars 3C 279 and PKS 1510-089. The PSDs are modeled as single power laws, and the best-fit spectral shape is derived using the "power spectral response"method. With more than 10 yr of data obtained with weekly/daily sampling intervals, most of the PSDs cover 1/42-4 decades in temporal frequency; moreover, in the optical band, the PSDs cover 1/46 decades for 3C 279 due to the availability of intranight light curves. Our main results are the following: (1) on timescales ranging from decades to days, the synchrotron and the inverse-Compton spectral components, in general, exhibit red-noise (slope 1/42) and flicker-noise (slope 1/41) type variability, respectively; (2) the slopes of 3-ray variability PSDs obtained using a 3 hr integration bin and 3 weeks total duration exhibit a range between 1/41.4 and 1/42.0 (mean slope = 1.60 ± 0.70), consistent within errors with the slope on longer timescales; (3) comparisons of fractional variability indicate more power on timescales ≤100 days at 3-ray frequencies compared to longer wavelengths, in general (except between the 3-ray and optical wavelengths for PKS 1510-089); (4) the normalization of intranight optical PSDs for 3C 279 appears to be a simple extrapolation from longer timescales, indicating a continuous (single) process driving the variability at optical wavelengths; and (5) the emission at optical/infrared wavelengths may involve a combination of disk and jet processes for PKS 1510-089.