BLT is a slur used to describe Blacks in a derogatory way. "Big-Lipped Tyrone," or for female version "Big-Lipped Tyreesha." Inspired from the 80's version of Degrassi Jr. High where the black guy wore a shirt with the phrase "BLT," his initials.
The word BLT is a racial slur that has been used historically to degrade and dehumanize blacks. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to blacks. If you are unsure about whether or not it is appropriate to use the word BLT, it is best to err on the side of caution and avoid using it in any situation.