  We carried out a high time-resolution, multicolour optical observing campaign
for eight $\gamma$-ray detected blazars during 2010-2020. We analyze flux
variations, correlations between magnitudes and colours on different
timescales. Intraday variability (IDV) is detected in all eight sources of our
sample. A bluer-when-brighter (BWB) chromatic trend is dominant on intraday
timescales. On the short timescales, the BWB trend only shows up in ON 231, 3C
279, BL Lacertae and 1E 1458.8+2249. There is a BWB trend in 3C 279 on the long
timescale. We estimate the upper limits of black hole mass for three blazars
(i.e. ON 321, 1ES 1426+42.8, PKS 1510-089) using variability timescales. On
April 13, 2010 a potential quasi-periodic oscillation (QPO) with the period of
$P=48.67\pm13.90$ minutes is found in 1ES 1426+42.8. The light curve on March
16, 2021 further shows the existence of the QPO phenomenon. The QPO in this
target deserves further observation and confirmation.
