1. A method for monitoring a blind spot of a vehicle, comprising steps of: (a) a blind spot monitor acquiring or supporting another device to acquire at least one feature map from a rear video image if the rear video image is acquired from a monitoring vehicle in a driving state; (b) the blind spot monitor performing or supporting another device to perform (i) a process of acquiring m proposal boxes corresponding to one or more objects located in the rear video image, (ii) a process of acquiring feature vectors each of which corresponds to each of the m proposal boxes by applying pooling operation to each area, on the feature map, corresponding to the m proposal boxes, and (iii) a process of inputting each of the feature vectors corresponding to each of the m proposal boxes into a first FC layer to acquire (iii-1) each of first classification scores for each of first kinds of class, which correspond to each of the m proposal boxes, to confirm whether the objects are monitored vehicles, and (iii-2) each of second classification scores for each of second kinds of class, which correspond to each of the m proposal boxes, to confirm poses of the objects; and (c) the blind spot monitor performing or supporting another device to perform (i) a process of selecting n proposal boxes, which have probabilities over a certain threshold to be regarded as including the monitored vehicle, among the m proposal boxes by referring to the first classification scores, (ii) a process of acquiring n bounding boxes, each of which corresponds to each of the n proposal boxes, by referring to regression information corresponding to each of the n proposal boxes, (iii) a process of determining the pose of the monitored vehicle corresponding to each of at least some of the n bounding boxes by referring to the second classification scores, and (iv) a process of determining whether a haphazard vehicle is located in a blind spot of the monitoring vehicle by referring to at least some of the n bounding boxes and the pose of the monitored vehicle, and wherein, at the step of (c), the blind spot monitor performs or supports another device to perform (i) a process of calculating first overlap ratios which are pairwise-overlapping ratios among the n bounding boxes, (ii) a process of determining specific bounding boxes confirmed to have the first overlap ratios equal to or greater than a first threshold among then-bounding boxes as corresponding to a single identical monitored vehicle, (iii) a process of calculating second overlap ratios which are pairwise-overlapping ratios among the specific bounding boxes and their respective corresponding proposal boxes, and (iv) a process of determining a certain bounding box with a maximum ratio among the second overlap ratios as including the single identical monitored vehicle.