  We present new theoretical period--luminosity (PL) and period--radius (PR)
relations at multiple wavelengths (Johnson--Cousins--Glass and {\sl Gaia}
passbands) for a fine grid of BL~Herculis models computed using {\sc mesa-rsp}.
The non-linear models were computed for periods typical of BL~Her stars, i.e.
$1\leq P ({\rm days}) \leq4$, covering a wide range of input parameters:
metallicity ($-$2.0 dex $\leq$ [Fe/H] $\leq$ 0.0 dex), stellar mass (0.5--0.8
M$_{\odot}$), luminosity (50--300 L$_{\odot}$) and effective temperature (full
extent of the instability strip; in steps of 50K). We investigate the impact of
four sets of convection parameters on multi-wavelength properties. Most
empirical relations match well with theoretical relations from the BL~Her
models computed using the four sets of convection parameters. No significant
metallicity effects are seen in the PR relations. Another important result from
our grid of BL~Her models is that it supports combining PL relations of RR
Lyrae and Type~II Cepheids together as an alternative to classical Cepheids for
the extragalactic distance scale calibration.
