Here's information about it:
Manufacturer & Pattern: It was made by the Macbeth-Evans Glass Company, an American company established in 1899, in their "Petalware" pattern.
Period: This specific cobalt blue glass version of the Petalware pattern was in limited production and dates back to the Depression era, specifically between 1930 and 1940.
Design & Use: The design features a distinctive petal pattern on the saucer and is often referred to as a mustard or condiment jar/cup due to its size and attached underplate.
Collector's Item: It's considered a collectible piece of Depression-era glassware and is sought after by collectors.