Hello, I have 2 things that come to mind first!
1) one option would be using a standard acrylic paint with a densely packed, soft brush. Dense (lots of bristles close together) will minimize the appearance of strokes, and soft material will blend gently (vs. a stiff brush that will leave a "scratchy" effect). Depending on the type of paint, it might need multiple thin layers.
2) is doing multiple soft/light layers in a pencil, marker, etc. For each different layer, you slightly change the direction of the strokes. Slightly changing the direction of the layered strokes is called cross-hatching. By doing this combined with multiple layers, it will trick your eye and you won't see the strokes...but it also fills in everything to be uniformly solid.
*Pro tip for pencil drawings (graphite and color)! to maximize a well blended look, never press so hard that it leaves it looking "shiny." If you tip your paper in the light and see the strokes reflect in the light, its been pressed too hard. To achieve a dark color but avoid the shine, you slowly add layer upon layer. It might seem tedious or impossible, but with practice you will get amazing results! Ideally, it should be completely matte when held to light.
I hope this helps! Good luck!