This in not blurriness, but the JPEG compression doing its thing. Strong colour contrasts between irregular shapes are always distorted like this by the compression in a .jpg. We call them ‘artefacts’.
You could try to reduce the amount of JPEG compression. Increase the ‘quality’ slider when you export / save as a .jpg and the results should be better. The higher the quality, the less compression artefacts and the larger the file.
If all else fails, try and export a PNG-24 instead, but be warned that most social media will re-save a .png as a .jpg anyway, and re-compress it. Reintroducing your problem.