Thanks. I did steps 1-5 successfully. On step 6 I power on and press select but same symptoms; blue light comes on and stays on but screens are blank. I don't see any of the messages so can't proceed with step 7.
One of the comments on that thread mention to get the boot.3dsx from hbmenu and add to add root. Tried that and same issue.
This sounds like your SD card slot reader can't get a good connection with the card you're using.
If you have canned air, spray the inner cavity of the card slot. You can also use a vacuum cleaner with small hose adapter to suck out dust or contamination. No canned air or vacuum cleaner? Use a plastic drinking straw to pull air;
do not push air as spit/saliva can corrode the pins! If you find dust clump in the slot, you can lightly prod those out with a wooden or plastic stick like a kebab skewer or toothpick.
Don't use metal.
For the SD card (assuming this an old 3DS series), try cleaning the golden contacts with q-tip and isopropyl rubbing alcohol. Due to CV and difficult getting that 70-91% isopropyl, you can substitute with water.
No cleaning the 3DS card slot reader with water or q-tip because the cotton fibers can get snagged and water can corrode the pins.
Try reseating the SD card by reinserting it at different angles. If cleaning the card and slot reader doesn't work, repeat those steps with a different SD card. Lastly, if all else fails, your 3DS card reader might be worn and needs to be replaced. Which exact model is this 3DS? [Old/New] [2/3] DS [XL] ??
Edit - Also try clearing the RAM with the
battery trick.
