Why did Blair feel the need to make a public announcement about his religious proclivities and why did he feel the need to wait until he was stepping down from public office to do it? Does he anticipate an appointment from the Vatican?

To avoid a constitutional crisis.

Why?

Erp: There would have been a constitutional problem because of the history of conflict between the Church and the criminal gang which had cut its way to power in England in the 16th Century. The history is summmarized herein:
http://www.catholic-history.org.uk/nwchs/recushandbook.htm

After the Papal Bull of Deposition, Regnans in Excelsis, this gang effectively excluded Romam Catholics from public life and from what we would consider normal civil rights. The Catholic Emancipation Act in the 19th Century restored some of these rights, but not the right to hold high office. The sources hold that a British Prime Minister would have to resign if he confessed the Faith.

The PM is essentially a monarch these days and given English history masy not be permitted to be Catholic, though it's the sort of thing they wisely don't test.

In enlightened and religiously tolerant Europe? I'm shocked, shocked to hear of such a thing.

Thanks Lou. Since PM Disraeli was a Jew, I had no idea there would have been a problem with a Catholic PM. Live and learn ... especially around here.

Disraeli wasn't even a crypto-Jew.

Was he C of E then?

Baptized even.

To repeat, live and learn.