(Please forgive the delay in response. Technical problems prevented the mailing of a number of responses.)

1) The Mefarshim speak quite a bit about why we mention Hatafas Dam Bris in the blessing for the Ger/servant, but I did not find anyone who explains why the addition of, "if not for the blood of the Bris, heaven and earth would not exist," is made.

2) It seems to me that since it was decided that "Dam Bris" must be mentioned in the Berachah (according to the Ramban -- because some Gerim received Milah before the Gerus so their primary Milah at the Gerus is Hatafas Dam Bris, and therefore a Takanah was made that Dam Bris should always be mentioned), it therefore was desirable to stress in the Berachah that it is not only Milah on which the heavens and earth stand, but the blood of Milah is also sufficient on its own to hold up the heaven and earth. We see from the words of the Berachah that even though the verse in Yirmeyahu talks about the Bris, this also means the blood of the Bris alone.

3) In other words, what the words of the Berachah are stressing is that one should not say that merely to draw blood of the Bris from someone who has already been circumcised is not considered a sufficiently important Milah to warrant saying a Berachah on it. Therefore, the Berachah stresses that the blood of the Bris alone is enough to keep heaven and the earth going.

4) Here is an additional insight for why this Berachah is made only for the Milah of a Ger/servant.

In fact it would be possible to say the Berachah about heaven and earth for everyone who receives a Bris. However, the reason is is not necessary to do this is that when we say the Berachos on the Ger/servant, this exempts us from making the Berachah about heaven and earth on any other people. Since the idea of heaven and earth is mentioned in the Berachah at least for one type of person, this covers our need to mention this aspect of Milah. The Berachah is more relevant for a Ger/servant for the reasons stated above, and this is why the need to mention heaven and earth is exempted with the Berachah for these people.

Kol Tuv,

Dovid Bloom