Block format is typically used for business letters. In block format, the entire text is left aligned and single spaced. The exception to the single spacing is a double space between paragraphs (instead of indents for paragraphs).
If you have been asked to complete a project in both block style and APA style, ask your instructor for clarification. It is likely that they want you to cite your sources using APA style and format the letter using block style.
*APA does use block quotations for quotes of 40 or more words, but this is something entirely different from block letter format. If you need information about block quotations, NOT block letters, visit: https://libanswers.goodwin.edu/faq/367969