i have the same problems as well sometimes....
i keep my bloods in melamine enclosures, and for the most part, the humidity stays in the range i like....but on cooler days with minute amounts of humidity in the air, it sometimes gets down to 40%....
I immediately spray them and get it up to 80% or so then it drops and the cycle continues....
I have not had any problems with my snakes getting RI's etc ....as of yet.
if it was possible, i'd have em at 60% year round....but unless u have an amazing facilty, with controled temps, it is nearly impossible to achieve this...
Assuming u get a CB, fluctuating %'s of humidity wont effect them that much...as long as they are not kept at inadequate levels for prolonged periods, they should be ok....
Keep em in rubbermaids for as long as u can, and then move them to melamine or plastic caging when larger....
If i was u, i would set up an enclosure which u would want to house them in, then buy a digital thermometer/hygrometer which has the min/max memorey for both....and test it for a week or so....
That way, u'll know how much it drops when ur not there....and maybe u could get a family member to spray the enclosure down when ur not home, etc...etc....
The digital thermo/hygrometer should only cost u about 35-40 bucks and is worth every penny when dealing with bloods.....
Its great to see ppl wanting to work with this species....they really are GREAT snakes....
Hope this helped....
Gvg