As the dew of Hermon, and as the dew that descended upon the mountains of Zion - This was not Mount Zion, ציון tsiyon, in Jerusalem, but Sion, שיאן which is a part of Hermon, see Deuteronomy 4:48; : "Mount Sion, which is Hermon." On this mountain the dew is very copious. Mr. Maundrell says that "with this dew, even in dry weather, their tents were as wet as if it had rained the whole night." This seems to show the strength of the comparison.
For there - Where this unity is.
The Lord commanded the blessing - That is, an everlasting life. There he pours out his blessings, and gives a long and happy life.
For other particulars, see the commentators passim, and the following analysis.