Blending - pictures where the background and foreground, or otherwise two separate items blend together due to having the same colour with no border to separate them.
Commonly done with the character's hair being the same hue as the solid colour backdrop.
A possible result of no lineart and flat color - see: Flat Color - Style
Recommended prompt structure:
Positive prompt:
blending, flat color, no lineart, negative space, {{tags}} {{color}} background, masterpiece, best quality, very awa, absurdres
Negative prompt:
(worst quality, low quality, sketch:1.1), error, bad anatomy, bad hands, watermark, ugly, distorted, censored, lowres, abstract, signature, bkub