The Blue Weimaraner has a distinct charcoal gray colored coat. Genetically speaking, Blue Weimaraners are dilute blacks and Grays are dilute browns.

Gray and Blue Weimaraner. The Gray Weimaraner is a dilute brown and the Blue Weimaraner is a dilute black.
The difference between a Blue and a Gray Weimaraner is in the tone of the color, not the deepness or darkness of the color. All Weimaraners, Blue or Gray, can be lighter or darker.
I’ve discussed the variation in colors of the Grays in my other article, “Not Just a Short Coated Gray Dog” but here are some more photos to illustrate, and in particular to compare to Blue Weimaraners.

You can see the various shades of gray in this photo. There is one Blue Weimaraner in the very back, but all of the other Weimaraners are gray.
Here is another example of light versus dark in a litter of Gray Weims.

These puppies are all gray; however you can see that there is a difference in the variations of gray.
Can you tell what color the puppies are in the photo below?

Difference in color and pigmentation in Weimaraners: Momma is gray. The two puppies in the front are silver-gray, the middle one (yellow collar) is gray, and the two puppies in the rear are both Blues.
What about this photo?

Two Blue Weimaraners and one Gray Weimaraner are pictured. As you can see, the Blue Weim in the blue collar is quite light, but the tone of her coat is different from the gray one who is wearing the orange collar.

These are the same dogs as the photo above. The gray one is older in this photo and in this photo, appears to have more of a brown cast.
Gray and Blue Weimaraners: Lighting Matters
Lighting can be deceptive, and it can make a big difference in seeing the true color!

The green collar puppy is a Blue Weimaraner; the orange collar puppy is a Gray Weimaraner. This photo is deceptive due to the lighting.

A very dilute Blue, appearing deceptively lighter due to all the light in the photo. Though she is light, the tone of her coat is still a diluted black rather than having any brown cast. Note her nose color is also affected by the coat genes showing a dark gray nose. (Darker Blues have black noses.)

The same dog (left) next to her littermate, both Blues.
The tell is the nose and rims of the eyes. Blue Weimaraners have noses and eye rims that are a very dark gray or black. Gray Weimaraners have noses and eye rims that are brown and/or fleshy color.
Easy Genetics
The genetics are quite simple; Blue is dominant to Gray, which means that you must have one Blue parent to have Blue puppies, and you can never get Blue puppies from two Gray parents. This is because if a dog is carrying a Blue gene, you will always see the Blue color. You can read more about genetics as well as information on color statistics based on mating on my other site, blueweimaraner.com.
A Short History on Blue Weimaraners
The Blue Weimaraner is basically an American “variety” that has a long and controversial history. There were few Weimaraners imported into the States prior to WWII, and it was after the war that a large number of imports were introduced to the American public via service men. Casar von Gaiburg, the first Blue Weimaraner was among them.
Casar was imported in 1949 and was issued his German papers in 1950 but several months later the German Club president wrote to the Weimaraner Club of America that Casar was cross bred. There has never been an answer to why the German Club changed their minds about Casar; they would not have issued his registration papers in the first place if they had believed he was cross bred. And thus started the controversy.

Cäsar von Gaiberg (“Tell”) is considered the “father of the Blues.”
Casar was AKC registered in 1950 based on his German papers, and therefore was considered purebred by the AKC. All his descendants, of both colors, are also considered purebred and are registerable. This acceptance did not quell the controvery. It raged on in this country, as both colors were being produced under a standard that allowed them. There were two unsuccessful attempts to change the standard to disqualify them prior to 1970. In 1971, the Weimaraner Club of America finally got the majority vote to disqualify the Blue Weimaraner, and this is the standard that is still in effect today.
Today we are over 65 years into the inter-mingled breedings of Blues and Grays. Blues are not rare!!! The are an AKC registerable purebred Weimaraner, but due to its rocky history, there can be some bias against the color. Blues are not allowed to be shown, and they are not accepted in other countries
Are Blue Weimaraners Different from Grays?
Blue and Gray differ in coat color only!
Bh
And Americans are still pushing the undesirable blue. You know these so called blue just dont exist in Germany. Americans trying to create another color of dog… That is a fact.
Anne Taguchi
While the Blue is an American variety, it is simply not true that Americans are trying to create another color of dog. The first Blue came from Germany. The Blue issue has a very complicated and very political history. It is documented here: blueweimaraner.com.
Jake Dempsey
Undesirable….. you’re oh so wrong.
I adore my blue. I also prefer the color. So unique
Otto
Ah another yahoo pushing fake news as “fact”. As mentioned numerous times Tell the single point of origin for the blues came from and was certified by Germany. We signed off on him but due to “political” pushback we tried to back track. It was a genetic anomaly but still pure. Stating otherwise is pure ignorance.
John Lang
The two blues in my family have different temperaments than any of the grays that I or my family have owned going back to 1984. It is not just a coloring difference. One originated from Tennessee and the other from the midwest. They are more willful and if a person I would say selfish or jerkiness. This does not mean that when people are present they are not loving and desire to please or any lack of intelligence. Our two blues have more of a retriever head than a pointers and do not have webbed feet nor do they have the South American aggressiveness. Other than one puppy all of our dogs have been rescue animals. I wish that you had mentioned in your article what the Germans in 1950 felt that Casar alleged cross breeding consisted of.