Per
http://www.merriam-webster.com/dictionary/absurd, "absurd" means "ridiculously unreasonable, unsound, or incongruous".
"so absurd as to be very happy in each other's company" means that Dr Block and his wife were very happy in each other's company, and they must have been very absurd people to feel that way.
The reason why the writer thinks this is "absurd" is hard to say just from this fragment. Perhaps they seemed an incompatible couple, perhaps they had disagreeable characters, or perhaps the writer is just making a wry observation that husbands and wives usually aren't happy in each other's company.