in an volume 82 sbs it was stated that one of blackbeards hobbys is historical research
and in the volume 88 sbs oda said that in the real world blackbeards real life career would be an archaeologist and robins role on the crew is the Archaeologist and she can read the poneglyphs
who else loves history but nico robin whose goal is to read the rio poneglyph and find out about the void century. its quite likely that blackbeard would like to know about the void century due to his hobby being historical research plus its been said that the poneglyphs contain info on the 3 ancient weapons which blackbeard would be interested in . plus there’s the road poneglyphs which contain the coordinates to raftel that iwhen all 4 are read you will get the coordinates to the location of raftel
so blackbeard knowing how to read poneglyphs would help him find raftel and make it so he wouldn’t have to kidnap robin for the road poneglyphs translation
and blackbeard has waited for years for the yami yami no mi and waited for years to betray whitebeard and obtain whitebeards fruit so that he would be able to become pirate king. so it would be fitting for him to went out of his way to learn how to read poneglyphs in order to be able to find raftel ( heck he could have learnt how to read poneglyphs on ohara before it was destroyed)
or he could have learnt how to read poneglyphs from kozuki oden who was on whitebeards crew
plus BB is a foil and evil counterpart to luffy. On one hand we have Luffy who dives head first into most situations (for better or for worse), and makes it through the grand line based moslty on sheer willpower and determination. On the other hand we have Blackbeard who takes the complete opposite approach, methodically plotting every step of his journey to becoming pirate king. First researching the yami yami no mi and spending years patiently waiting for it to show up. Then after obtaining it through a stealthy assassination (almost exact opposite of how luffy fights ) ,he continues his plan of capturing ace, becoming a shikibukai, busting shillew out, and then on top of that stealing whitebeards fruit. I almost have to respect the insane level of thought he puts into his actions and the results he achieves.
so it would be fitting if while luffy just stumbled upon the person with the ability to read poneglyphs and help him on his quest along with himself sharing rogers voice of all things ability which would allow him to hear the voices of poneglyphs without any study blackbeard has learnt how to read poneglyphs through studying