  A spectral variation accompanied with flux variability is a commonly-observed
phenomenon for blazars. In order to further investigate the optical spectral
feature of blazars, we have collected the long-term optical V and R band data
of 27 blazars (14 BL Lacs and 13 FSRQs), and calculated their optical spectral
indices. The results show that the spectral indices vary with respect to the
brightness for all of these blazars. In general, the optical spectrum
progressively becomes flatter (or steeper), when the brightness increases.
However the spectrum changes more and more slowly, until it tends to be stable.
In other words, the source becomes bluer (or redder) and then gradually
stabilizes when it brightens, which are briefly named the
bluer-stable-when-brighter (BSWB) and redder-stable-when-brighter (RSWB)
behaviors, respectively. Thirteen of the 14 BL Lacs show the BSWB behavior,
with an exception of AO 0235+164. On the contrary, most of FSRQs (10 out of 13)
exhibit the RSWB trend. It is confirmed that blazars follow the two universal
optical spectral behaviors, namely, BSWB and RSWB. The model of two
constant-spectral-index components can well explain the optical spectral
features qualitatively and quantitatively. The results illustrate that the
optical emission are mainly composed of two stable-color components, i.e., less
variable thermal emission and high variable synchrotron radiation. And in most
cases, the thermal component of BL Lacs is redder than that of synchrotron
radiation, whereas FSRQs are the opposite.
