A blessing is an auspicious formula. It has thus the same purpose as a benediction (prose) or invocation (metrical), but a blessing is a short, formulaic phrase.
Blessings in Tamil manuscripts
There are standard blessings, ubiquitous in Tamil manuscripts, such as hariḥ om (usually Sanskrit written in Grantha), civamayam, tirucciṟṟampalam. Such blessings have thus a strong sectarian flavour as they appear to be sorts of credo.
Blessings are found in the margin of the leaf, along with the title or intertitle, but also in-text, in intermediary completion statements at the end of chapters or in colophons.

Marginal tirucciṟṟampalam blessing, along with title and intertitle. Bnf Indien 334. Courtesy of BnF.

In-text tirucciṟṟampalam blessing, at the end of a chapter (after the chapter completion statement and the total verses). Bnf Indien 983. Courtesy of BnF.
It appears that the blessing is connected to the copyist of the manuscript rather than to the content of the manuscript. Thus one can find in the manuscript of a Śaiva text a supposedly Vaiṣṇava blessing (hariḥ om) supposedly by a Vaiṣṇava copyist.
Other blessings, such as naṉṟāka, kuru vāḻka or kuruvē tuṇai, seem more ecumenical, while sectarian variants, such as civa-kuruvē tuṇai, are found.
The X-tuṇai (“X is the refuge / the help”) is particularly productive and accommodated to various contexts and texts.

Final (end of manuscript) blessing tīruccīt=tampalam [i.e. tirucciṟṟampalam]. Bnf Indien 95. Courtesy of BnF.

Intermediate (end of chapter) blessing śrīrā=maceyam and tirucciṟṟampalam. Bnf Indien 96. Courtesy of BnF.