I just looked in Ansel Adams' The Negative and several other websites, and found that the spectral response of blue only film matches almost perfectly to the 47B filter. The film died out at 500nm, which is where the 47B cuts all light. The response of the filter suggests that at 430nm a 1 stop adjustment in exposure is required, getting higher as the color shift away from its "center," allowing only 0.17% of light at 500nm. The 47 is sharper cutting at the 340-430nm end, but higher over the rest of the blue end.

I hope this will help. B and H has the filters reasonably priced, and I'm sure you can find it elsewhere cheaper!