Can I leave a Kugel wrapped in silver foil on the blech over Shabbos or is it Hatmana?

You may.

Hatmana is for the purpose of keeping or adding heat. The silver foil is for the purpose of practicality and cleanliness etc. If that purpose was already achieved and extra layers of foil are used, it will indeed be an issue of Hatmana.

ע”פ מ”ש בשו”ע אדה”ז סימן רנז ס”ד.