I'm sure it has to do with the fact that less people are getting married. In the 1940's, it was scandalous to have a child out of wedlock; today, it isn't.
I'd love to see your source, because even with the lower marriage rate, 70% seems rather high. I'd also like to see statistics on the white illegitimacy rates of the past and present. I guarantee you the white illegitimacy rate has at least doubled.
As far as conservatives blaming the black illegitimacy rate on "heavy government aid", less that 50% of black Americans receive aid, so (according to you) there's a higher illegitimacy rate, than the rate of blacks receiving aid - so that doesn't add up.
You state that it obviously isn't racism, but I beg to differ. Why are these conservative only concerned about the black illegitimacy rate, when the illegitimacy rate for all people has gone up? When you single out and complain about the illegitimacy rate in one race, and ignore the identical behavior in all others, that sounds like someones being racist to me.