  We present an analysis of the spectral properties observed in X-rays from
active galactic nucleus BL Lacertae using RXTE, Suzaku, ASCA, BeppoSAX, and
Swift observations. The total time covered by these observations is
approximately 20 years. We show that this source undergoes X-ray spectral
transitions from the low hard state (LHS) through the intermediate state (IS)
to the high soft state (HSS) during these observations. During the RXTE
observations (1997 -- 2001, {180 ks, for a total 145 datasets}), the source was
approximately 75%, 20% and only 5 of the time in the IS, LHS, and HSS,
respectively. We also used Swift observations (470 datasets, for a total 800
ks), which occurred during 12 years (2005 -- 2016), the (0.3 -- 200 keV) data
of BeppoSAX (1997 -- 2000, 160 ks), and the (0.3 -- 10 keV) data of ASCA (1995
-- 1999, 160 ks). Two observations of Suzaku (2006, 2013; 50 ks) in
combinations with long-term RXTE and Swift data-sets allow us to describe all
spectral states of BL Lac. The spectra of BL Lac are well fitted by the bulk
motion Comptonization (BMC) model for all spectral states. We have established
the photon index saturation level, Gamma_{sat}=2.2+/-0.1, in the Gamma vs. mass
accretion rate (Mdot) correlation. This Gamma-Mdot correlation allows us to
estimate the black-hole (BH) mass in BL Lac to be M_{BH}~3x10^7 M_sol for a
distance of 300 Mpc. For the BH mass estimate, we use the scaling method taking
stellar-mass Galactic BHs 4U~1543--47 and GX~339--4 as reference sources. The
Gamma-Mdot correlation revealed in BL Lac is similar to those in a number of
stellar-mass Galactic BHs and two recently studied intermediate-mass
extragalactic BHs. It clearly shows the correlation along with the very
extended $\Gamma$ saturation at ~ 2.2. This is robust observational evidence
for the presence of a BH in BL Lac.
