My jalapenos have always done just so so. This year I planted them in those black tree containers and they've done very well. It's probably the heat that those containers retain.
I think you are right about the black containers holding heat.
I grow jalepanos only to make salsa. My tomatoes are late this year and I ended up giving away something like 60 of them to DD's coworkers. I treated all my peppers the same this year, dug good holes after deweeding and thoroughly tilling, threw compost at the bottom, mulched around one group (with no seed first year burdock cut and mulched with my bag mower) and THAT has kept them from drying out, and giving them good watering to start with then nothing. I also used tomato cages for all of them and I have taken some time to train them from escaping the cages and growing upwards. We've had some good heat, and most are producing.