Any mingling (i.e., even of straight whiskey of one type and even without addition of a blending agent), can smooth out a palate to be sure. This can occur even when a company mingles from its own stocks of a given type. I believe the smallness of the industry in Maryland - really small compared to PA and especially KY - was the main factor for companies to rely on blends of straight whiskey and blends proper as their mainstay.
However, we should note that straight rye was sold in Maryland of course. Apart from the original (post-Pro) Pikesville, there were numerous brands in the market as Bready's article notes. In the Melrose history alone, are mentioned Canton Maryland Straight Rye Whiskey, American Straight Rye Whiskey, Old Record Bottled in Bond Rye Whiskey. Also listed are two brands of blends of straight whiskeys (one all-rye) and two blended whiskeys per se. All these were products of the Melrose company. Did sales of the blends exceed the straights? I do not know but suspect they did. Head of the list in the book was Melrose - Blended Straight Rye Whiskies - clearly its flagship. Clearly though this was not a hugely popular category; if it was, there might be straight rye made in Maryland today.
The straight ryes of Maryland in my opinion were made with all-rye or mostly rye.
Gary