front caliper bleeder blocked?

If one of the bleeders on each caliper bleed fine you should be fine on the bleed. If you rebuilt the calipers and the caliper is dry then you might need to access the 2nd bleeder. If your brakes work you are good. Test carefully after any brake work. You can remove a bleeder and clean if out with wire. The caliper will drip and as long as you don't take all day and drip until the master is exposed you will be OK. Just bleed that caliper and you good. The inside bleeder is rarely bleed. It's the one that usually clogs from disuse.

 

I wouldn’t put a bolt in there because there’s a taper in there and you might damage it.

You can work on the individual caliper using a brake hose clamp. That will stop the flow and allow you to do what you need to without re-bleeding the entire system. Just stick “brake hose clamp” into Amazon, personally I have switched to plastic clamps.

If you don’t want to use a hose clamp, eg. you have braided hoses and don’t want to dent the braiding then you can use an old-school trick.

That is to remove the brake fluid filler cap and seal the reservoir filler neck with some plastic so that it is totally airtight, you will slow the flow of brake fluid to almost nothing. Then remove the bleed nipple and plug the hole with an appropriate bung. Then you can go off and clean the bleed nipple. Once finished remove the bung, replace the bleed nipple, remove the plastic from the reservoir, and then bleed just the caliper that you were working on, and you’re good to go.

But if it were me, I’d just use a brake hose clamp.

 

if it's seeping through the threads, you should be able to just remove the bleeder, stick a plastic plug or similar in the caliper while you clean it, then put it back in. As long as you put something in there to prevent fluid from just dripping out it should be fine, even if if slowly leaks.