In general, the Black Lagoon anime adheres closely to the manga. It captures the key elements and the gritty atmosphere of the source material. Some scenes might be presented differently for better visual storytelling, but these changes are relatively minor and don't stray far from the manga's core.
The Black Lagoon anime ends around a certain point in the manga, but it's not an exact one-to-one match. There are variations and additional storylines in the manga that weren't covered in the anime.
The Black Lagoon manga is not ongoing anymore. It wrapped up some time ago, but it left a lasting impression on fans with its exciting storyline and memorable characters.