Rose Gentle, whose son Gordon was serving in the 1 Royal Highland Fusiliers, said the fact sensitive extracts of exchanges between the then Prime Minister and US president are to be held back would fail to answer many of their questions.
Years of negotiations over the release of the "vital" material, which includes 25 notes from Mr Blair to Mr Bush and more than 130 records of conversations between the men in the run-up to the war, have caused delays to the Chilcot report into the invasion.
Under the agreement that has now been thrashed out, the information being disclosed will be limited to "quotes or gists" and the inquiry's use of the material "should not reflect president Bush's views".
But Mrs Gentle, of Pollok, Glasgow, said: "It's not surprising really [they will remain secret], but the families are a bit disappointed because these letters are the most important thing to let us understand why we did go into Iraq."
Her son was blown up by a roadside bomb as he travelled in an armoured LandRover in 2004, but she still has many questions about the justification for the conflict and wanted to know what Mr Blair and Mr Bush were saying to each other at the time.
She added: "When we are talking about people's lives, like my son's life, we should have the right to see these letters."
The deal was struck by the government's top civil servant Sir Jeremy Heywood, who was principal private secretary to Mr Blair at the time, and inquiry chairman Sir John Chilcot.
In a letter, Sir John told the cabinet secretary: "I appreciate that the inquiry's requests for disclosure raised difficult issues of long-standing principle, which have taken some months to resolve: recognition of the wholly exceptional nature of this inquiry has allowed that to happen.
"My colleagues and I judge that this material is vital to the public understanding of the inquiry's conclusions."
Sir John said some "potential gaps" in material had been identified which had now been addressed, "including some material received by the inquiry very recently".
The inquiry completed public hearings in 2011 and there is criticism over the delay in releasing the report. The publication date is yet to be agreed.
Mr Blair insists he was not the reason for the delay and called for the letters to be published.
A Cabinet Office spokesman said: "The government is pleased that agreement on a way forward on both cabinet papers and UK/US exchanges has now been reached with the inquiry.""