If you're speaking to the blond secretary in question, then, ".תודה, מזכירה בלונדינית"
/to'da lax, maski'ra blon'dinit/
If you're talking about her with someone in third person, then, ".תודה למזכירה הבלונדינית"
/to'da lamaski'ra hablon'dinit/
thumb rule:
thanks -- תודה
thank you -- תודה לך