When you say, "randomly generated", all the recipients signed up for this, right? It's not completely random? If it were completely random, that would be wierd and creepy to get something like that out of the blue.
If everyone is a member of this postcard club, then I think it would be fine/nice to offer general words of support for all the scary news lately. I think anything TOO specific about the Boston bombing could have the boomerang effect of making the recipient feel slighted, if they are in the middle of some other more local issue (such as the Texas explosion).
For example, when the superstorm Sandy was all over the news media, I found it very upsetting, because the year before my region had been absolutely devasted by tornados - there were entire towns that just disappeared, not two sticks standing together. Those storms killed 10x as many people as Sandy and left many rural people homeless and without power, water or fuel. So the news coverage of Sandy left a bitter taste in a lot of people's mouths down here, because it just reinforced that nobody cares what happens to us.
I know that if somebody down here had gotten a message saying "thinking of you because of all the devastation from Sandy", it would not have been well received.