You made a mistake in putting it on a blog, that is one of the drawbacks of the internet ? whatever you write on there is essentially available for anyone to read. But it was your dp who withheld the information from your dsd, and in reality it is his fault that she found out from her friends because if he?d been honest with her in the first place none of this would have happened.
It?s not as if his dd was conceived and then forgotten, he has regular contact with her and she knows about her half sister yet the half sister doesn?t know about her, I think your dsd has every right to be upset, and as it?s your blog that the info came from it?s natural that it?s you she?s upset with.
IMO your dp is probably annoyed with you because he knows that by not telling her she?s found out in the worst way imaginable, and that once her anger towards you blows over she will very likely be resentful towards him for not telling her in the first place.
Now that it is all in the open, you can all look to making things right, and hopefully incorporating his dd2 into your lives as well perhaps.
All you can do is be there for your dsd, apologise to her that she found out in the way she did, but IMO you don?t owe your dp an apology because it?s because of his dishonesty this mess ever happened, after all if all this secrecy wasn?t going on you surely wouldn?t have had the need to write about it in your blog in the first place.