While I agree with you that there is a 130-year period of LDS history (1848 - 1978) where blacks were heavily discriminated against, I cannot say that black people today should not join the Church.
The Church today is a very different organization than in 1848 when the Priesthood Ban went into effect. Brigham Young was, for lack of a better term, a racist bigot and incorporated his feelings into the doctrines of his day, despite Joseph Smith's open approval (and ordination) of blacks to the Priesthood and Temple. Other LDS leaders continued the tradition for the sake of not contradicting Brigham until the 70's, when Spencer W. Kimball finally said "Now wait a minute... how did this actually come into effect?" and lifted the ban.
Since this time, the membership and leadership has gradually become less racist, and I can honestly say that the last quote from an LDS leader with racist overtones (at least that I am aware of) was just over 10 years ago, in 1996, when they last discouraged interracial marriage.
Despite its historical black eyes, the LDS Church has been an influence of good for many people, and even if it isn't "True" the way it claims to be, we cannot be the judge of others who wish to either join or leave it.