Poppy seed (or poppyseed) dressing is often associated with Helen Corbitt, the Dallas cookbook author and Neiman Marcus food manager in the 1940s and 1950s. She admitted that she helped to popularize poppy seed dressing in Texas, but she said that she’d come across it when she was in New York City.
 
 
26 September 1947, Dallas Morning News, section II, pg. 5:
POPPY SEED DRESSING.
Mix 1 1/2 cups sugar, 2 teaspoons dried mustard, 2 teaspoons salt, 2/3 cup white vinegar, 2 tablespoons onion juice and stir until sugar is dissolved.
 
Beat mixture very fast as 2 cups salad oil are slowly added. Beat until thick and clear. After beating thoroughly add 3 tablespoons poppy seeds. Store in refrigerator. As plenty of beating is most important in making this dressing tasty, it is made best in an electric mixer.
 
4 October 1950, New York Times, pg. 49:
Poppy Seed Salad Dressing
For fruit salads, a poppy seed dressing is made by Mary K. Behlen’s of Cambridge, Mass. Available in Brooklyn at Abraham & Straus and Ecklebe & Guyer,  De Kalb Avenuem it comes in a seven and one-half ounce bottle for 49 cents.
 
Bits of poppy seed suspended in the vegetable oil add crunchiness as well as interesting flavor to this salad dressing. Vinegar and sugar are blended to give a sweet-sour savor—with the sweet a little more pronounced.
 
18 December 1966, New York Times, pg. 70:
JOHNSON CITY, Tex., Dec. 17 (AP)—Mrs. Lyndon B. Johnson stocked up on hill country delicacies at a food sale today, buying more than her red and green shopping basket would carry.
 
The First Lady spent $12.75 at the sale, in Johnson City’s recently completed park, for such home prepared foods as pickled okra, poppyseed salad dressing, fig preserves and a rarity called agarita jelly.
 
The Best from Helen Corbitt’s Kitchens
edited by Patty Vineyard MacDonald
Number 1 in the Evelyn Oppenheimer Series
Denton, TX: University of North Texas Press
2000
Pg. 122:
I cannot get by without Poppy Seed Dressing though I’m personally tired of it. Why wouldn’t I be? Years and years of watching it consumed by customers. Everyone likes this dressing on practically every kind of fresh salad. I recommend it for fruit.
 
Where it originated I have no idea. I remember having it served to me in New York so many years ago I hate to recall. Rumors extend hither and yon that I created it; I hasten to deny this; but I did popularize it when I realized that on the best grapefruit in the whole wide world (Texas grapefruit) it was the most delectable dressing imaginable. Today there is hardly a restaurant or home in Texas that does not have some kind of poppy seed dressing. The recipe I use has been in demand to the point of being ludicrous and, strange as it may seem, the men like it—a few even put it on their potatoes. So here it is!
 
POPPY SEED DRESSING
3 1/2 cups
 
1 1/2 cups sugar
2 teaspoons dry mustard
2 teaspoons salt
2/3 cup (white) vinegar
3 tablespoons onion juice
2 cups vegetable oil (but never olive oil)
3 tablespoons poppy seeds
 
Mix sugar, mustard, salt and vinegar. Add onion juice and stir it in thoroughly. Add oil slowly, beating constantly, and continue to beat until thick. Add poppy seeds and beat for a few minutes. Store in a cool place or the refrigerator, but not near the freezing coil.
 
It is easier and better to make with an electric mixer or blender, using medium speed, but if your endurance is good you may make it by hand with a rotary blender. The onion juice is obtained by grating a large white onion on the fine side of a grater, or putting in an electric blender ,then straining. (Prepare to weep in either case.) If the dressing separates, pour off the clear part and start all over, adding the poppy-seed mixture slowly, but it will not separate unless it becomes too cold or too hot.
 
{To keep dressing from being grainy, heat the vinegar and sugar together until the sugar crystals dissolve; cool the syrup and proceed [Illegible copy].
 
Pg. 123:
One of my most popular buffet salad bowls at the Houston Country Club, where I was manager, was finely shredded red cabbage, thinly sliced avocado, and halves of fresh grapes with Poppy Seed Dressing—but then Poppy Seed Dressing fans like it on anything.