Hi @percypea, glad your new boy is doing better.
x2 on @casportpony's suggestion to keep a close eye on him. Sometimes blackhead (histomoniaisis) can lie hidden as the bird's immune system keeps it more or less in check, then can erupt when the bird goes through a period of stress. As you mention, getting caught (particularly for a bird which is not frequently handled), stuffed in a box, traveling and having to adjust to new buddies are all stressful events, as is a change in temperature different from the temperature to which the bird has been accustomed.
Stress all by itself can cause the bird's poo to change, even without a disease organism -- we've all experienced the immediate poo (mostly worn it, lol) from a just-caught, frightened and upset pea
To make a long story short, yes, it is possible for them to have blackhead and not be showing much in the way of symptoms. Do you have appropriate medications on hand? Metronidazole plus an antibiotic for the secondary bacterial infections.
Also, since the two birds are new to your flock, you will certainly want to worm them (even if the seller has done so, or has said he/she has done so). A five day regimen of fenbendazole, followed by another 10 days later, would be wise. You can use ivomec spot on to get rid of any external parasites such as lice, but don't count on it for getting rid of the worms. Regular worming with a wormer such as fenbendazole is your peas' best defense against blackhead, by keeping the worms which carry the histomonas protozoa at bay. Never follow any instructions which call for putting fenbendazole in water unless it is the special aqueous formulation of it -- we can't get that version easily here, but it is more available in the UK I think. It is new and should make life easier. If you need help with dosage amounts, we can help, or you can do a search here on the peafowl forum -- it is a frequently answered question.
Stress can also cause an outbreak of coccidiosis in an adult bird, so you will also want to have some amprolium on hand as well as something like dimethox, sulmet or trimethoprim.
Congrats on your new boys! Now you just need girls...