From the NPS site on the Confederate powder works at Augusta, Georgia
https://www.nps.gov/nr/travel/Augusta/sibleymill.html:
"The Confederate Powder Works, the only permanent edifice constructed by the Confederate States of America, was in operation until April 1865. During its lifetime, the facility produced approximately 7,000 pounds of gunpowder per day for a final total of 2,750,000 pounds. The Augusta Powder Works produced enough gunpowder to fully meet the needs of the Confederate armies and still retained a surplus of 70,000 pounds at the end of the war."
It would be very impressive if they were supplying powder by means of niter beds before the middle of the war, because basic chemistry would militate against it.
Since the history of the Confederate powder works states this:
"The caves of Arkansas were rich in nitrous earth, and those of Texas still more so, and these supplied the armies west of the Mississippi river with material for gunpowder. As early as practicable I sent out instructed powder-makers to both those States, who under the directions of the military authorities, assisted to put up the necessary powder mills for the Trans-Mississippi department, which after the fall of Nashville was left necessarily to its own resources.
In the early part of November my time had become so much occupied that it was no longer practicable to attend to the production of saltpetre, and Mr. F. H. Smith was sent from Richmond by the Chief of Ordnance to relive me from its duties. At a later day a separate department was established, called the Nitre and Mining Bureau, which then had the entire charge of its production.
In the latter part of November, by the desire of General Lovell the able officer in command at New Orleans I proceeded to that city and examined the temporary arrangements for making gun powder, and also conferred with him relative to procuring a supply of saltpetre from abroad. He suggested the chartering of the steam ship Tennessee, then lying idle in the river near the city, to proceed at once to Liverpool and take in a cargo of saltpetre and return to New Orleans, or, in case of necessity, to put in at Charleston or Wilmington. The suggestion met my views, and was approved by Mr. Benjamin, then Secretary of War, but was not carried out on account of the effective blockade of the mouth of the Mississippi.
The Confederate Government, however, by its agents in Europe, purchased saltpetre which was shipped on swift blockade runners which arrived from time to time at Charleston and Wilmington. This proved to be adequate to our wants, and about two millions, seven hundred thousand pounds were thus received during the war and sent to the Confederate Powder Works. The amount obtained from the caves amounted to about three hundred thousand pounds for the same period. Thus the total amount received at the works amounted to about 1,500 tons."
- General George Rains,
History of the Confederate Powder Works
What it looks like was going on is that the Confederates imported saltpetre from Europe by blockade runner and turned it into powder at the Confederate Powder Works. I've seen a reference to suggest the amount of domestically produced niter in the CSA was on the order of a couple of hundred tons from niter beds and (as per above) about 150 tons from the caves; thus about 80% or more of all CS powder was made with British niter.
I apologize for having conflated the import of powder with the import of the main constituent of powder; indeed most imports were of nitre and the powder was manufactured in America. But the effect would be much the same - almost-total shutdown of warmaking for want of powder.