Hi there armiiiiu,
We understand you're seeing a blue pixel stuck on your Apple Watch, and we'd like to see if we can assist.
First, restart your Apple Watch, and check for the issue again: How to restart your Apple Watch
If the issue persists, try changing the background or watch face: Change the watch face on your Apple Watch
This article has some great information regarding pixels: About LCD display pixel anomalies for Apple products released in 2010 and later
"In many cases pixel anomalies are caused by a piece of foreign material that is trapped somewhere in the display or on the front surface of the glass panel. Foreign material is typically irregular in shape and is usually most noticeable when viewed against a white background. Foreign material that is on the front surface of the glass panel can be easily removed using a lint free cloth. Foreign material that is trapped within the screen must be removed by an Apple Authorized Service Provider or Apple Retail Store.
If you are concerned about pixel anomalies on your display, take your Apple product to an Apple Authorized Service Provider or Apple Retail Store for closer examination. There may be a charge for the evaluation."
You can contact Apple Support or use this link to find a nearby location: Find Locations
Cheers!