Wotan,
I agree that a slower Allied bomber is needed. I well remember having a Boston Mk III run my Bf109E-4 down. It irritates me when the Allied players whine about the Ki-67 and completely ignore how impossible the Boston Mk III is for the Axis to deal with.
However the massively fragile Blenheim has much less defense than the Ki-67 (perhaps you overlooked the two 12.7mm machine guns in the tail), a top speed of 265mph with only 1,000lbs of bombs. I'd have no objection the the Blenheim light bomber being added, but not at the expense of the Allied side having a usable bomber for the 1940 and 1941 time frame.
My suggestion for that would be a Vickers Wellington Mk III medium bomber. The Wellington had a reputation for being very durable, but it only has a top speed of 255mph. It was armed with eight .303 machine guns, two in the nose turret, four in the tail turret and one in each beam position. It also had a usable payload of 4,500lbs. The Wellington meets the requirments of being a usable, yet sufficietly vulnerable, early war Allied bomber.