The black section of the interstage is improved thermal protection added with the Block 5 version of the Falcon 9, just to increase booster reusability since the second stage engine often charred it a bit.
The Falcon Heavy core is constructed very differently to a Falcon 9, however. The interstage features the upper struts connecting the two boosters to the core, which you can see folding inward on separation, so structurally it needs to accommodate that assembly and also bear the heavy stresses imparted by the boosters themselves. This likely meant there wasn't room for the improved thermal protection in design, and since it's far less critical, it was left out.
The main Block 5 features incorporated with the Falcon Heavy are the improved engine performance, grid fins, and landing legs.