The choice of how to format a block quote has nothing to do with grammar. That is more in the domain of typographical conventions. In general, with stylistic matters, there is no single right answer because there are many different ways to format text.
It wouldn't exactly be "wrong" to place quotation marks around a block quote; it would just be highly unusual and it would stand out to most readers. That is bad because you would generally want the readers to be focused on the content, not on the formatting. Hence, the appropriate style should be chosen based on the audience.
If the document is a court filing, the court's rules may specify a particular style to be followed. For example, the BC Court of Appeal has published
a practice direction which specifies that papers submitted to the court are to comply with a particular named style guide, with certain modifications as described in the practice direction. If submitting a paper to the BC Court of Appeal, you would want to comply with those instructions, regardless of whether you like them and regardless of whether they are consistent with what you find on a given "grammar website".
If in doubt, you may want to ask your firm what style guide they adhere to for the particular kind of communication in question. If there is no specified guide, then you would look at what is common in other communications of that type. For example, if you are writing a letter to a client, you may want to look at what kind of formatting your firm has used in letters to clients in the past. If there is no guide to be followed, the overriding principle is just to avoid doing anything that the reader will consider to be wrong or unusual, because if the reader finds the formatting jarring, that will detract from the content.
