GlowBlock

GlowBlock draws colored outlines around blocks but only around the ones you've actually looked at.
Walk past an ore vein and it stays marked, so you can find your way back to it later without keeping a mental map.

Everything runs on the client. No server-side installation, no permissions needed.

How it works

The mod scans your surroundings for blocks on your tracking list and then performs a line-of-sight check on each one. A block is only marked once enough of its visible surface was genuinely exposed to your camera.
Blocks buried behind stone, hidden around a corner, or submerged in lava never get picked up. Once a block is marked, the outline stays until you break it or remove the block from tracking.

Features

• Per-block colors -> give every tracked block its own outline color
• Color presets -> save color sets and switch between them
• Block picker -> browse and pick blocks through a searchable in-game menu
• RGB editor -> dial in exact colors with sliders, or pick from quick presets
• Distance fade ->  outlines fade out gracefully toward the render limit
• Per-dimension memory -> marks are kept separately for Overworld, Nether and End

Usage

Open the main menu with the configurable keybind

Note 

GlowBlock does not reveal anything you couldn't already see.
It has no wall-vision, no chunk scanning for hidden ores, and no server-side component  it simply remembers what was in front of your eyes.
That said, some servers restrict any form of block highlighting, so check the rules before using it on a Server.