Something that is fortuitous happens by chance or without known cause. In modern standard use, the term almost always carries the senses both of accident or chance and luck or fortune, as in the case of a fortuitous encounter with a former acquaintance that leads to a promising new career opportunity (a happy accident!). Beware that some stylists and editors may object to the use of fortuitous to mean anything more than "accidental." In formal writing, when your intended meaning is less about a happy accident and more about fortune's favor, the adjective fortunate may be a better option.
A bland sauce is mild and tasteless. A bland vocabulary lacks flavorful descriptors such as insipid! Insipid means without distinctive, interesting, or stimulating qualities, as in the case of remarks that are uninspired and flat, or song lyrics that are so clichéd they are utterly dull. Neither of these terms is used as a compliment, generally speaking, but insipid is the more pointed of the two, connoting at times a shallowness or lack of mental rigor.
To allude is to refer to something indirectly. It is a way of pointing toward something or calling it to mind without explicitly naming it. A pop singer might allude to events in their personal life through abstracted lyrics. Allusions, or references to earlier artistic works, are very common in literature, as in the case of a book title (Brave New World by Aldous Huxley) that borrows language from another work (The Tempest by William Shakespeare).