You should have gone with your first instinct.
He wore a t-shirt that said “BLM” on it. He might have worn it because he agrees with the specific aims of one of the legally contracted public organizations that use BLM in their names. He might have worn it because he agrees with the plain meaning of the phrase, “Black lives matter.” He might have worn it because he agreed with the other sentiment displayed on the shirt - which we’ll get to in a second - and the shirt happened to have been produced by a BLM-affiliated organization.
That imagery has been common in Civil Rights arguments for decades.
“It demands great spiritual resilience not to hate the hater whose foot is on your neck, and an even greater miracle of perception and charity not to teach your child to hate.” - James Baldwin, 1963
“That’s not a chip on my shoulder, that’s your foot on my neck.” - Malcolm X, 1964
“I ask no favor for my sex, all I ask of our brethren is that they take their feet off our necks” - Ruth Bader Ginsburg, 2018
MLK, of course, was a Civil Rights leader who spoke often about the evils of white supremacy, and who was, famously, frequently the target of police violence.
While the shirt designer likely was referencing Floyd’s murder by changing “foot” to “kneeling,” it’s a leap to go from that t-shirt message, to the idea that the person wearing it was too biased to sit on this jury. You will be hard pressed to find a Black person who does not think that Black lives matter, or who does not think that police violence should go unaddressed: and that’s as much as can be learned from the photo of the gentleman at the MLK rally. If the bar for keeping a Black person off a jury in a civil rights case is that low, you’re effectively arguing that no Black person can sit on a jury for a civil rights case.
You have not provided even a smidgen of evidence that this is true.