Bloomsbury was incorporated as a borough by an Act of the New Jersey Legislature on March 30, 1905, from portions of Bethlehem Township.
The Borough of Bloomsbury was once known as "Johnson's Iron Works", owned by Robert Johnson, on the north bank of the river. The current name is derived either from thE Bloom family, influential in the early history of the town, or from the iron ore processed into "blooms," masses of wrought iron.
As of the census of 2000, there were 886 people, 322 households, and 252 families residing in the borough. The population density was 975.5 people per square mile (375.9/km²). There were 342 housing units at an average density of 376.6/sq mi (145.1/km²). The racial makeup of the borough was 98.19% White, 0.34% African American, 0.23% Native American, 0.34% Asian, 0.11% from other races, and 0.79% from two or more races. Hispanic or Latino of any race were 1.47% of the population.
There were 322 households out of which 45.7% had children under the age of 18 living with them, 66.1% were married couples living together, 9.0% had a female householder with no husband present, and 21.7% were non-families. 15.8% of all households were made up of individuals and 5.3% had someone living alone who was 65 years of age or older. The average household size was 2.74 and the average family size was 3.11.
In the borough the population was spread out with 29.8% under the age of 18, 3.8% from 18 to 24, 38.4% from 25 to 44, 17.9% from 45 to 64, and 10.0% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 93.0 males. For every 100 females age 18 and over, there were 93.8 males.
The median income for a household in the borough was $64,375, and the median income for a family was $67,500. Males had a median income of $51,053 versus $33,750 for females. The per capita income for the borough was $26,392. About 4.8% of families and 3.8% of the population were below the poverty line, including 3.1% of those under age 18 and 8.9% of those age 65 or over.