Howdy,
The US Army's 1850 Ordnance Manual described the infantry accoutrements (For muskets) as essentially the same as since 1841, as worn through the war with Mexico... including the 1.5-inch wide waist belt with the small US oval plate...
The riflemen's accoutrements, were similar, but lacked the shoulder belt for the cartridge box... and the waist belt was 2-inches. These accoutrements were for troops armed with rifles instead of muskets.
The Army changed its uniform in June, 1851. The plates depicting the uniform dress of the infantry troops showed the infantry accoutrement belts to be thereafter "officially" black...
Here's some sketches of the infantry at Fort Snelling Minnesota in 1851 by Frank B. Mayer, yet in the pre-1851 fatigue dress, and Mexican war style 1841/1850 pattern infantry accoutrements...
Here's a regular army infantry sergeant in the early 1850s, with the post-1851 black sword belt, worn with the "old pattern" fatigue dress from the Mexican war. Though notice his rank chevrons are on a dark blue backing, as for the regulation post-1851 dark blue frock coat...
It took the Army time to produce and issue the new 1851 pattern uniform caps and coats, etc. The pre-1851 dress was continued, including the sky-blue fatigue jackets and dark blue cloth forage caps, though was to be modified, so far as was practicable, with the new branch colors...
Within a short period, ca. the sky-blue fatigue jackets for infantry were modified, with the white tape removed, and post-1851 branch colors, dark blue for infantry, added as piping...
Anyways, the blackening of the white infantry accoutrements was not attended to universally or instantaneously. Augustus Meyers states that upon enlisting in 1854, at the recruit depot at Governor's Island, New York, the serving troops in garrison were yet employing the white buff belts...
Here's an infantryman in the 1851 uniform with an unblackened infantry waist belt (narrow)...
Meyers states that his 2nd US Infantry Regiment, received rifles and rifle accoutrements after 1855...
...
The 7th Infantry Regiment, serving in the Southwest, was not rearmed in the 1850s apparently, and as late as 1861 still had the smoothbore percussion muskets (pattern of 1842).
C&D Jarnagin sells the Mexican War period accoutrements, including the 1841 Musket Cartridge box (for .69 caliber musket), with small, 1839 pattern box flap plate, and the corresponding belts and the narrow waist belt with small waist belt plate, etc.
CC Sutlery has some inexpensive accoutrement accessories of the type (imported). Also the correct tin canteen (with the US embossed in center... though with an incorrect strap... as it should have a russet leather one)...
NEW C&C Sutlery Mexican-American War (1846-1848)
www.ccsutlery.com
Missouri boot and shoe sells pre-1861 type leather canteen slings...
Buy A Single U. S. Gun Sling, Canteen Strap or Bayonet Scabbard OR Buy A Combo & Save On Shipping NOTE: For Complete U. S. Accoutrements See Our U. S. Accoutrements Page
www.missouribootandshoe.com
Missouri Boot and shoe also sells a good quality 1839/41 pattern musket cartridge box...
FEATURES INCLUDED AT NO EXTRA CHARGE Hand Applied Period Finish Hand Made C artridge Box Tins Are Included At No Extra Charge Hand Stitching with correct thread and stitches per inch All hardware is...
www.missouribootandshoe.com
Consider getting the Mexican War accoutrements in the white buff, since they weren't necessarily blackened immediately in every case; and if wearing them in black to reflect the regulations is preferred, blackening them yourself, etc.