In this case, I agree with NorthernMama in saying that, from the conditions you're describing, I'd leave the blanket on.
A little trick that, so far *knocks on wood*, has helped me know if a blanket is enough or not is looking at the leg fur on my kiddos. If they're in a blanket and their leg fur is poofed out to where their legs look like bottle-brushes, I stick my hand in the blanket and feel. If my hand is warmed, then I figure they're good.
If their legs are poofed out like that and it's cool under their blanket, I figure that they're cold so I step up the blanket situation.
If their legs aren't poofed out at all and they look "slick", plus being warm/hot under the blanket, I figure they're too hot so I take the blanketing down a notch. And so forth - legs not poofed but under their blanket is slightly warm and it's raining, I figure they're fine...etc.
So far this hasn't failed.
Of course, I rarely blanket in the day - unless it's windy and below freezing. We get some fierce winds over here so windchill can take 10+ degrees off the temperature, plus the whole "wind cutting through the winter coat"-thing.
Same thing with rain - it's rare that I won't blanket in the rain, just because I prefer my horse to go out vs standing in his stall to keep his precious lil' hiney dry.
But, overall, my goal is to blanket as little as possible.