Thom,
I had informed Mrs. Black upon receipt of the invitation that I did not think my health would permit me to attend. It seemed the easiest route, given my lack of engagement with society of late, and in truth I suspected—rightly—that simply wouldn't have it in me to pretend I wanted to be there.

I do hope my absence will not prevent you from having a jolly time. Please give my best wishes to Mr. and Mrs. Black, and extend once again my regrets.
With affection,