I'll not speak for PastoralMusings, but to the general subject of the use of the name Missionary Baptist.
Broadly, it goes back to a controversy over mission boards, auxiliaries, etc. that divided Baptists in the United States in the early part of the 19th century (roughly 1820-1840). Those who favored missionary societies or mission boards were classed as "Missionary Baptists," while they usually called the other side "anti-missionary" (but those groups called themselves Regular Baptists, Old School, Primitive, etc.). The name still falls generally in that realm, though it is often used by groups and/or churches that are not in favor missionary societies or mission boards like in the SBC or ABCUSA. The "Missionary Baptists" split over the way to do missions -- societies, board missions, Gospel missions, direct missions, etc. You usually just have to just figure out how it is used the geographical area where you see it. In our area "Missionary Baptist" means churches that are pro-missionary but not affiliated with the Southern Baptist Convention. When they split here in the early 1900s, the non-SBC side was usually in the majority in the local associations and took the name "Missionary Baptist." If you visit East Texas you will quickly find that a church with the name "Missionary Baptist" is either a predominantly white church that is not in the SBC, or a predominantly African-American church that is affiliated with one of the National Baptist Conventions. I think, but don't hold me to it exactly, that this will generally be true in Oklahoma, Arkansas, Louisiana, and Mississippi -- and probably Tennessee and Missouri. In my visits to northern Alabama and northern Georgia, I have found that if a church had "Missionary Baptist" in the name it was usually Southern Baptist.
Hope this helps.