A complete blessing admits no deficiency

וה' ברך כו' בכל.

"And Hashem blessed Avraham with everything (ba-kol)" (Bereishis 24:1).

The Sefas Emes addresses the blessing that Avraham was blessed "with everything."

עתה נתקיים הבטחה של ואברכך.

Now the promise of "and I will bless you" (Bereishis 12:2) was fulfilled.

This "blessing with everything" is the realization of the earlier promise Hashem made to Avraham at the start.

ונ"ל כי לא ברכו הקב"ה קודם לידת יצחק בעבור שהי' עתיד עשו לצאת ממנו.

And it appears to me that Hashem did not bless him before the birth of Yitzchak, because Esav was destined to come forth from him.

The blessing was withheld until after Yitzchak's birth — because had it come earlier, it would have flowed into the line from which Esav would eventually descend.

ומה"ט לא בירך הקב"ה ליצחק ג"כ עד אחר לידת עשו.

And for this reason Hashem also did not bless Yitzchak until after the birth of Esav.

Likewise, Yitzchak's own blessing was delayed until after Esav was already born and separated out.

כי אם הי' נתברך בכל לא הי' יכול לצאת ממנו שום דבר חסרון:

For had he been blessed "with everything," no deficient thing could have come forth from him.

A complete blessing of "everything" leaves no room for any flawed offshoot; so the blessing waited until the flawed branch (Esav) had already emerged, leaving only the wholly blessed line to carry it forward.

Summary: Avraham's being "blessed with everything" fulfilled Hashem's original promise of blessing — but it was deliberately withheld until after Yitzchak's birth, and Yitzchak's until after Esav's. Since a blessing of "everything" admits no deficiency, it could not be given while the flawed line of Esav was still to emerge; only once that branch had separated out could the complete blessing rest upon the pure, chosen line.