You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Race_and_ethnicity_in_the_United_States_Census

Context:
The 1850 census saw a dramatic shift in the way information about residents was collected. For the first time, free persons were listed individually instead of by head of household. There were two questionnaires: one for free inhabitants and one for slaves. The question on the free inhabitants schedule about color was a column that was to be left blank if a person was white, marked "B" if a person was black, and marked "M" if a person was mulatto. Slaves were listed by owner, and classified by gender and age, not individually, and the question about color was a column that was to be marked with a "B" if the slave was black and an "M" if mulatto.

Question:
In the 1850 US census, what was put into the color column for black persons?

B