According to the first sentance, in the first paragraph of the volume's opening essay called "Text, Translation and Elucidation" the ArtScroll's Saperstein edition of the Torah "attempts to render the text as Rashi understood it." In other words, they translate according to Rashi.
Here's Rashi on the first appearence of the word va'yivorech:
"This is a greeting, in the manner of all who appear before a king... saluder in Old French [and, as the notes add, saluer in Modern French.]"In the translation, however, we're told: "...and Jacob blessed Pharoh."
This error is avoided in the Gutnik edition, (another house's rival translation which also promises to "translate according to Rashi") where the first use of va'yivorech is translated as "greet" and the second, per Rashi, is translated as "blessed." So where did ArtScroll go wrong? Two speculative answers:
1 - They were sloppy. The word va'yivorech means blessed in common Hebrew. In fact, Robert Alter, in his Five Books of Moses, reads it as "blessed" both times and adds "...it would be entirely in keeping with his own highly developed sence of his patriarchal role that he - a mere Semitic herdsman chief addressing the head of the mighty Egyptian empire-- should pronounce a blessing..." It's not at all unreasonable to translate it this way-- unless you've made a point of announcing that you will follow Rashi.
2 - Ideology got in the way. Readers of the ArtScroll Stone edition of the Torah (a translation with an anthology of popular exegetes) know that whenvever ArtScroll chooses between two interpretations, they almost always go with the one that is most fanciful. In the Stone, Serach lived forever, Rivka was three on her wedding day, and the second Egyptian plague began with one frog --even though there are other classic commentators in each case who saw things less magically. It's perfectly in keeping with this editorial approach for ArtScoll to present Yaakov, the "mere Semitic herdsman chief" in this more exalted way-- even though this isn't how Rashi saw it.