You can improve the print, but you can’t recover detail that isn’t in the file. For a low-resolution, blurry image, focus on making it look better at print size rather than trying to create true sharpness.

Useful approaches mentioned:

• Upscale carefully: try enlarging gradually or use a good resampling method such as Lanczos. Photoshop’s Preserve Details and tools like waifu2x or PhotoZoom were also suggested.
• Sharpen after resizing: use Unsharp Mask or Smart Sharpen gently. If blur is severe, a mild pre-sharpening pass may help before final enlargement, then do output sharpening at the end.
• Add texture/grain: a little grain or texture can improve perceived sharpness in black-and-white prints. Printing on matte paper or canvas can also help hide softness.
• Consider stylized output: halftone or vector/engraved-style treatments can work well if the image is too soft for a natural-looking enlargement.

Best practice: make a test crop at the final print size before ordering the full print. That will show whether the image looks acceptable from normal viewing distance.