Nyala Park is a private wildlife area contained within the large sugar estates not far from Chikwawa in the lower Shire Valley. As you are driving south on the M1 between Chikwawa and in the direction of Nsanje, the turn off to Nyala Park is on the left not far from the turn-off towards Lengwe National Park.
Nyala Park has giraffe and some other wildlife not found in the main National Parks and Game Reserves in Malawi. For this reason it is worth a visit in combination with a visit to Lengwe National Park. It is also worth a stop if you have time when passing for other reasons on the M1.
It is NOT worth a trip from as far distant a location as Blantyre on it's own (unless you plan to camp and enjoy being in the natural environment there). The reason it's not worth a long journey on it's own is that it is too small and after you have driven around for about 50 mins you will likely have seen all you are going to see.
I do recommend it however in combination with something else. It's quite something to see a giraffe towering over you if that's something you've not experience before.