His saying: "Blessed is He (Tabaraka)" is from the concept of blessing (baraka). "In Whose hand is the dominion (al-mulk)" means: in His hand is the dominion.
قَوْله: ﴿تبَارك﴾ هُوَ مِنْ بَابِ الْبَرَكَةِ ﴿الَّذِي بِيَدِهِ﴾ أَيْ: فِي يَدِهِ ﴿الْمُلْكُ﴾
