Here's another thought:
An overcast morning at EI 40 should give you about f:5.6 at 1/40 sec., so make it 1/30 sec. to account for the magnification factor, and adding 2.5 stops for filter factor gives you f:4.5 at 1/8 sec., so you should have been in the ballpark, exposure-wise.
Could you post a scan? Like Donald, I'm a bit unsure about what you mean by "blown out midtones."