Black audiences in the early 70's loved Bruce Lee. He was popular among white moviegoers too, but he was especially well-loved by young black moviegoers. He was a non-white leading actor, and like blaxploitation films, his kung fu movies were revenge fantasies where an outsider challenged the establishment.

The high box office returns of Hong Kong movies like Five Fingers of Death in black neighborhood cinemas was noted at the time. Kung fu films were so popular that grindhouse cinemas in these areas would often show a kung fu movie and a blaxploitation movie together as a double feature. It did not take long for the makers of blaxploitation films to realize that adding kung fu sequences into their films would be popular with their audiences.

In 1973 Warner Bros financed Bruce Lee's last film Enter The Dragon. Jim Kelly was cast in a supporting part. Kelly was a martial arts instructor who had trained several celebrities. He was also black, good looking, and a decent actor---he'd played a few small roles in action films before. The movie was hugely popular and helped propel the the Black Fu movie trend throughout the 70's, with Jim Kelly starring in several of them.

My source on this:

China Forever: The Shaw Brothers and Diasporic Cinema by Poshek Fu. This book is fascinating if you are into classic kung fu films at all. It is a series of essays written by different people: the essay called Black Audiences, Blaxploitation and Kung Fu Films, and Challenges to White Celluloid Masculinity specifically addresses your question.

This is an amazing question. There are a number of factors that led to the prevalence of kung fu tropes in '70s blaxploitation flicks, which I will try to outline. My best source for this is Sundiata Keita Cha-Jua's article "Black Audiences, Blaxploitation and Kung Fu Films, and Challenges to White Celluloid Masculinity".

The first one is the internationalization of the civil rights movement and the transition to the black power movement. In 1967, when Martin Luther King Jr. made his opposition to the Vietnam War abundantly clear, it represented a major shift that was occurring in the civil rights movement: civil rights leaders were expanding their view to address American foreign policy. King's long-held belief that "injustice anywhere is a threat to justice everywhere" was being made manifest, and it damaged his relationships with many supporters of the civil rights movement who felt that he should be focusing exclusively on domestic inequality and racism—LBJ, for one. This was certainly not the first time that a black leader had expressed solidarity with other oppressed non-white peoples in foreign countries, but I bring it up as an important example of how the civil rights movement in the late 1960s was beginning to address the ways in which capitalism and colonialism were connected to domestic racism.

Eventually, this current of thought would become more radical and explicit in the views of the Black Panther Party and other black nationalists. The Panthers are relevant here because they were (a) Marxist and therefore internationalist (as opposed to "cultural nationalists" like the Nation of Islam), frequently expressing solidarity with non-white peoples around the world (like, say, in Asia) and with radical members of the Asian American Movement; (b) heavily influenced by the writings of Mao Zedong and therefore very interested in China and the structure of its society; and (c) opposed to King's doctrine of nonviolence, encouraging African-Americans to arm themselves for self-defense (and, in some cases, receive martial arts training) rather than submit to white violence, a theme that would prove central to blaxploitation. Their influence on African-American culture would create fertile ground for the embrace of kung fu motifs among black audiences (as well as blaxploitation motifs: Cha-Jua notes that "blaxploitation films utilized Black Nationalist tropes and codes to appeal to the militant sentiment permeating black communities").

The second big factor is probably just the fact that kung fu and blaxploitation movies happened to hit America around the same time. Sweet Sweetback's Baadasssss Song, from 1971, was the first hit film to feature the characteristics associated with blaxploitation, though some argue that it doesn't fall into the genre itself due to being independently produced. (This is probably the right spot to mention that the relationship between blaxploitation films, real life, and black and white audiences is a subject of fierce debate among scholars of film and of African-American studies.) Shortly thereafter, the style was picked up by major studios and the era of blaxploitation began in earnest. Meanwhile, the genre of hyper-violent kung fu films can be traced to the Hong Kong film studio Shaw Brothers, who in the mid-1960s began to articulate a reaction to the supernatural, mystical wuxia films (think Crouching Tiger, Hidden Dragon, basically) that were popular at the time. In 1973, Warner Brothers brought the Shaw Brothers' Five Fingers of Death to the US, where it became the first big kung fu hit. Later that same year appeared the earliest film that you linked to as an example of blaxploitation/kung fu mixture: Enter the Dragon. Black audiences lapped it up, and so, as with the original blaxploitation phenomenon, the studios started to make more films featuring this particular fusion. Warner Brothers, which distributed films from both genres, began to screen double features with one of each at inner-city theatres, a formula which proved successful.

Why did black audiences find this combination so appealing? For one thing, Cha-Jua cites film scholar David Desser's argument that kung fu and blaxploitation films shared non-white protagonists enacting righteously justified violence against white or colonialist foes. Kim Hewitt, in her article "Martial Arts is Nothing If Not Cool: Speculations on the Intersections Between Martial Arts and African American Expressive Culture", argues that the idea that "any individual can use his body as a weapon if he trains hard enough... immediately appeals to individuals who have been denied physical and social power historically and on a daily basis." Crystal S. Anderson, discussing Enter the Dragon in her book Beyond The Chinese Connection: Contemporary Afro-Asian Cultural Production, notes that the gangster's island where the tournament occurs is a sort of transnational place (claimed by both the British and Chinese governments) that serves as a site of escape for similarly transnational characters, black, white, and Asian. (Jim Kelly's character flees the US after justifiably attacking a pair of white cops in a scene straight out of a blaxploitation film.) This kind of thing shows up in a number of Lee's films—Cha-Jua calls it his "polycultural politics." In the case of Enter the Dragon, at least, it was certainly a conscious attempt by the film's producers to bridge the gap between Lee and the American audiences who were unfamiliar with him—a much more capitalistically (some might say exploitatively) motivated sort of internationalism than that of the Panthers.

I hope this was helpful; I certainly had fun writing it. If you want to read even more about this than my incredibly long-winded explanation, definitely check out Cha-Jua's article.