I have this exact car seat... it isn't too bad, I think what makes it a little hot is all the cushy padding, not the color since it's always in the shade in the car anyways (we put it in the center back). We live in Maine, so it hasn't been super hot like the rest of the country! But, I do just put him in it in just a diaper when it's been really hot, he gets so sweaty!
Hope that helps!
ETA: I should add that I absolutely love the seat overall, if you are considering it... color and all:)
Re: britax boulevard in onyx
I have this exact car seat... it isn't too bad, I think what makes it a little hot is all the cushy padding, not the color since it's always in the shade in the car anyways (we put it in the center back). We live in Maine, so it hasn't been super hot like the rest of the country! But, I do just put him in it in just a diaper when it's been really hot, he gets so sweaty!
Hope that helps!
ETA: I should add that I absolutely love the seat overall, if you are considering it... color and all:)