So here is a quick workaround to play Assassins Creed 1 backup on 3.55 / 3.41 PS3.
It's really dirty, there could be other ways but this one works so here it is.
First of all, grab this EBOOT.BIN EBOOT.BIN
This is just the original AC1 (EUR) EBOOT where I changed the first dev_bdvd string to look for dev_hdd0/GAMEZ/AC/USRDIR/ instead of dev_bdvd/PS3_GAME/USRDIR (ill explain about this later) and then I signed it with usual geohot tool.
I was too lazy to make a pkg for you, so just make a pkg with a random title id (not AC1 id, else it wont install data properly) with that EBOOT or install any pkg and put that EBOOT via FTP in hdd0/game/YOURTITLEID/USRDIR, as you wish.
Then, you need to either copy the USRDIR directory of AC1 to hdd0/GAMEZ/AC/ (NOT ANOTHER FOLDER !!! so you have a USRDIR dir in hdd0/GAMEZ/AC) or make a shadow copy of that folder with this special multiman from deank (multiMAN_SLINK_TEST_UPDATE.rar). In that case, you will need to have AC1 backup on hdd0, to create a shadow link to hdd0.
Then you can reinstall either the latest multiman you are using or use Gaia manager to load AC1 backup like any backup you would load from a backup manager. And it will go to XMB, as usual, but DONT START the game from the Bluray disc off XMB, else it will black screen as usual. Instead, boot the game from the package you installed previously using that special EBOOT, and voilą the game will work !!
There is no way to make it work 100% from XMB pkg, you need to use a backup manager to mount it to bdvd, else the game will quit after trying to install data, saying it's a dirty disc. So you need to do that and either use a shadow copy, or copy the game TWICE to your hdd (yeah...).
Finally, about a thoery about why some games like AC1 black screen... Well as you can see, I only changed one string about dev_bdvd, and still the game will use data from dev_bdvd (this string is only about TOC reading it seems) since I left all the other strings unchanged. If you don't change this string, and boot it from hdd0, the game will black screen as usual, so it means the issue isn't related to backup managers way of BOOTING the game, but of MOUNTING the game. The game simply can't access at some level the bdvd and thus crash (in the case of AC1, it cant find the TOC, but can find the data properly).
I didn't try to do that with other non working games, but I'm pretty sure it s the same issue.
In the end, most black screen games could be properly fixed if we somehow redirect THE "buggy" dev_bdvd string to dev_hdd0 or dev_usb, so it looks instead in these and we dont need to edit EBOOTS.
Finally, yeah people will say it's basically the "XMB pkg method". It kinda is, because the game still uses the mounted disc at some level to install it and load datas, so it doesnt look 100% for it in hdd0. This game simply doesnt work if you change all strings to hdd0 anyway, so here is a fix for this game, since nobody managed to get it to work.