  It has been found that in mixtures of nematic E7 and smectogenic cholesteryl
oleyl carbonate (COC) the S$_A$-N* transition temperature is substantially (by
~20 K) increased, as compared with pure COC, at E7 concentrations around ~40%.
Within the same concentration range, the isotropic transition is preceded by
formation of blue phase, with its maximum width of ~3.5 K clearly correlated to
the increased thermal stability of the S$_A$ phase. With other cholesterol
esters or cyanobiphenyls (cholesteryl nonanoate, 5CB), this effect was either
much weaker or not observed. No enhancement of smectic phase was noted when E7
was replaced by N$_{tb}$-forming nematic mixture based on CB7CB with two
cyanobiphenyl moieties. Selective Bragg reflection of light (BRL) spectra were
measured in all three temperature regions, including the unwinding of the
cholesteric helix on cooling towards S$_A$ phase and characteristic selective
BRL changes in the blue phase. In the latter case, the measured
{\lambda}$_{max}$ values were dependent both on the helical twisting power in
the cholesteric phase and on the lattice size and orientation in the blue
phase. Also considered were the effects of ferroelectric BaTiO$_3$
nanoparticles accumulated at disclination lines upon the blue phase thermal
stability.
