I think you only sing that for vigils and saints "with Polyeleos", not other times; so, it's not an omission.
I'm not sure you're correct. The PDF to which Shlomlokh
posted a link is for Great Vespers of the Sunday of Orthodoxy, to be read this Saturday evening. According to the Russian tradition I know, "Blessed is the man..." is sung as the Kathisma for all
Saturday Vespers services, as well as for the Vespers of many feasts. I acknowledge the possibility that Antiochian practice is different, but if not, it's readily apparent to me that this omission of the Saturday evening Kathisma is indeed a "blip in the system". In fact, this perception is further confirmed by Mother Mary and Metropolitan Kallistos Ware in their translation of the Lenten Triodion. In their section for the Sunday of Orthodoxy, they offer this introduction to Great Vespers on Saturday Evening:
After the Psalm of Introduction (Psalm 103) and the Great Litany, we read Blessed is the man (the first kathisma of the Psalter).