This is one of the Targum's most surprising explanations. Pseudo-Jonathan on Genesis 25:11 asks the question the Torah leaves hanging: why, in all the final chapters of his life, did Abraham not formally bless Isaac?
The Targum's answer is quietly brilliant. "Because Abraham had not designed to bless Ishmael, therefore he blessed not Isaac; for had he blessed Isaac and not Ishmael, it would have kept them in enmity."
Read it again. Abraham withheld a blessing from the chosen son in order to protect the relationship between the chosen son and the one who was not chosen. If he had publicly blessed Isaac and pointedly skipped Ishmael, the grievance would have calcified into a lifelong feud between brothers. So Abraham stayed silent. He let God handle the blessing after he was gone.
And God does. The next clause: "But, after the death of Abraham, the Lord blessed Isaac; and Isaac dwelt near the well at which was revealed the glory of the Living and Eternal One, who seeth and is not seen."
Isaac did not lose a blessing. He received it from a higher source, in a higher way, at Beer-lahai-roi — the well where God is known as the Seer. And crucially, the blessing came after Abraham's death, too late for Ishmael to feel disinherited in his father's own ceremony.
There is wisdom here that applies to every household. Sometimes the most loving thing a parent can do is not perform a public preference. Let the favored blessing come from God, in God's time, in a place apart. Abraham chose the harder path of silence, and kept his family, if not reconciled, at least not further wounded.