  Invisible plasma content in blazar jets such as protons and/or thermal
electron-positron ($e^{\pm}$) pairs is explored through combined arguments of
dynamical and radiative processes. By comparing physical quantities required by
the internal shock model with those obtained through the observed broadband
spectra for Mrk 421, we obtain that the ratio of the Lorentz factors of a pair
of cold shells resides in about $2\sim 20$, which implies that the shocks are
at most mildly relativistic. Using the obtained Lorentz factors, the total mass
density $\rho$ in the shocked shells is investigated. The upper limit of $\rho$
is obtained from the condition that thermal bremsstrahlung emission should not
exceed the observed $\gamma$-ray luminosity, whilst the lower limit is
constrained from the condition that the energy density of non-thermal electrons
is smaller than that of the total plasma. Then we find $\rho$ is $10^2$-$10^3$
times heavier than that of non-thermal electrons for pure $e^{\pm}$ pairs,
while $10^2$-$10^6$ times heavier for pure electron-proton ($e/p$) content,
implying the existence of a large amount of invisible plasma. The origin of the
continuous blazar sequence is shortly discussed and we speculate that the total
mass density and/or the blending ratio of $e^{\pm}$ pairs and $e/p$ plasma
could be new key quantities for the origin of the sequence.
