When we talk about a (good) person who is no longer alive we tend to add the phrase "Zecher Tzaddik Livracha" and we are used to the abbreviation, even in English - zt"l! Generally we translate this as "Of Blessed Memory" I always understood this as meaning his memory or Neshama should be blessed. Or some people say "May his memory be a blessing."
Upon reading the opening Rashi of the parasha, it occurs to me that this phrase originally could have refered to those who are living as well! Certainly Rashi and earlier texts apply it in a very different manner than its contemporary usage.
Lets remind ourselves. The parsha starts with the verse:
ט) אֵלֶּה תּוֹלְדֹת נֹחַ נֹחַ אִישׁ צַדִּיק תָּמִים הָיָה בְּדֹרֹתָיו אֶת הָאֱלֹהִים הִתְהַלֶּךְ נֹחַ:
וַיּוֹלֶד נֹחַ שְׁלֹשָׁה בָנִים אֶת שֵׁם אֶת חָם וְאֶת יָפֶת
Obviously, the question is that teh text begins saying: "These are the generations (offspring) of Noach" but it fails to mention his sons until the next verse! Why the interjection of Noah's righteousness? After all, it would seem to be a very different point!
Rashi tries to explain:
ט) אלה תולדות נח נח איש צדיק - הואיל והזכירו ספר בשבחו, שנאמר (משלי י ז) זכר צדיק לברכה.
"Once the Torah raises Noah's name, it tells of his virtue. As it states: The memory of the righteous should be blessed."
It would appear that the logic is as follows. Whenever a positive person comes up, say a little of his good deeds, his achievments.After all, we are dealing with a Tzaddik! How can we mention him without at least a story, an indication of his good actions?!
Hence, the minute Noah's name is mentioned, the Torah has to say the "bracha" i.e. that he was righteous and walked with God, and then it can get back to the point of his children.
In other words Zecher Tzaddik Livracha is "When you mention a Tzaddik, state his good points." Or possibly "When you mention the name of a Tzaddik, state that he is blessed i.e. a person of blessed virtuous status."
The original verse comes from Mishleiמשלי פרק י פסוק ז
זֵכֶר צַדִּיק לִבְרָכָה וְשֵׁם רְשָׁעִים יִרְקָב:
"The mention of the Tzaddik should be for a blessing and the name of the Rasha (evildoer) should rot"
and Rashi comments there:
(ז) זכר צדיק לברכה - המזכיר צדיק מברכו
If you mention a Tzaddik, bless him.
The verse in context is quite clear. The name of evil people should rot, decompose, be forgotton. The righteous should not only be mentioned but should be blessed, enhanced, publicised, celebrated.
So far, what we have is that when we bring up a good person in conversation (also a living person), we should stop and mention his greatness or bless him.
The Mishna is a little closer to our usage. In the Mishna of Yoma, there is a historical list of "good" indiciduals and "bad" priestly families. The "bad" families kept their skills to themselves refusing to teach and share their expertise. For example one family knew the method of the blending of the incense but kept the skill within their family. I imagine this was so they could remain influential as they were essential to the working of the Temple. The good families all donated special objects for public use. After listing the families, the Mishna concludes:
על הראשונים נאמר זכר צדיק לברכה ועל אלו נאמר ושם רשעים ירקב (משנה יומא פרק ד :
in other words, let the memory of these bad families rot and let the good deeds of these people be eternally remembered for good. On the one hand this is close to our zt"l, as we are remembering personalities of the past for their good deads. On the other hand it is more than "of blessed memory."
I imagine that we reached our contemporary meaning quite simply. Every time the name of a Tzaddik would be mentioned, rather than actually detaiing his good acts, people would just quote the words of the passuk in Mishlei (like the Mishna in Yoma) and hence people simply use the phrase זצ"ל. For Rashi however, it could certainly apply to the living as much as for the dead.